Pronoun : - A pronoun is a word that can replace a noun in a sentence. The noun that is replaced by a pronoun is called an antecedent . Examples are 'it', 'she', 'something', and 'myself'.

Indefinite Pronoun: are words that refer to persons or things in general and not to anything specific.

A pronoun that refers to person or thing in a general way (not in a definite way) is called indefinite pronoun. The indefinite pronouns everybody, anybody, somebody, all  each, every, some, none, one do not act for specific nouns, but function themselves as nouns.

Indefinite pronouns may be used without antecedents. e.g.

   (a) One cannot get everything one desires.

   (b) I will try to think of somebody who can help you .

How Do you use them ?

1.  You use an indefinite pronoun when you are not referring to any particular person or thing :

• There's something struck to the bottom of my shoe.
• Are there enough seats for everybody ?

2.  The pronouns anything and anyone/anybody are often used in place of something and someone/somebody in questions and negative sentences :

• I thought I heard someone call my name. Is anyone there ?
• I need something to wear to Jane's wedding. I don't have anything smart enough.

3.  You can put an adjective after an indefinite pronoun :

• Did you meet anybody interesting at the party ?
• Nothing definite has been decided.

4.  The indefinite pronouns are singular. You use them with a singular verb, for example :

• Is anybody going to meet you at the station ?
• No one makes better curry than you do.

5.  However, you may use the plural pronouns they, them and themselves, and the plural determiner their to refer to someone/somebody, anyone/anybody, everyone/everybody and no one/nobody :

• Somebody has left their coat behind.
• Can everyone see themselves in the mirror ?

6.  Some people prefer to use he or she, him or her, himself or herself and his or her to refer to these indefinite pronouns :

• Somebody has left his or her coat behind.
• Can everyone see himself or herself in the mirror ?

In the question : -

I will try to think of somebody who can help you. In this sentence , somebody is the correct indefinite pronoun as it is used to show a person who is not known or mentioned by name .

Thus , the Correct Option is B , which is somebody .

Pronoun : -  any of a small set of words (such as I, she, he, you, it, we, or they) in a language that are used as substitutes for nouns or noun phrases and whose referents are named or understood in the context’ .

Indefinite Pronoun: are words that refer to persons or things in general and not to anything specific.

A pronoun that refers to person or thing in a general way (not in a definite way) is called indefinite pronoun. The indefinite pronouns everybody, anybody, somebody, all  each, every, some, none, one do not act for specific nouns, but function themselves as nouns.

Indefinite pronouns may be used without antecedents.

Example : - Are there enough seats for everybody ? Here , everybody is an indefinite pronoun as it is used to refer to people which is not specific or define .

In the question : -

1. Has everyone completed the work that was assigned for today? In this sentence , everyone is an indefinite pronoun as it is used to refer to people which is not specific .

2. I don't have anything smart enough. In this sentence , anything is an indefinite pronoun as it is used to refer to thing which is not specific .

3. Somebody has left his or her coat behind. In this sentence , somebody is an indefinite pronoun as it is used to refer to people which is not specific .

Thus , the Correct option is C , which is everyone , anything , somebody .

Pronoun : - Pronouns are words that take the place of nouns. We often use them to avoid repeating the nouns that they refer to .A pronoun is a word that is used instead of a noun or noun phrase. Pronouns refer to either a noun that has already been mentioned or to a noun that does not need to be named specifically.

Indefinite Pronoun: are words that refer to persons or things in general and not to anything specific.

A pronoun that refers to person or thing in a general way (not in a definite way) is called indefinite pronoun. The indefinite pronouns everybody, anybody, somebody, all  each, every, some, none, one do not act for specific nouns, but function themselves as nouns.

Indefinite pronouns may be used without antecedents.

Example : - One cannot get everything one desires. Here , everything is an indefinite pronoun as it is used to specify about the things which are not definite .

In the question : -

__________ can make a mistake.You should not be over confident.

In this sentence , anyone is the correct indefinite pronoun as it  is used to specify about the people which is not definite .

Thus , the Correct  Option is A , which is Anyone.