Pronoun : - A pronoun (I, me, he, she, herself, you, it, that, they, each, few, many, who, whoever, whose, someone, everybody, etc.) is a word that takes the place of a noun. In the sentence shiva saw shaina, and he waved at her, the pronouns he and her take the place of shiva and shaina, respectively.

Relative Pronoun :-  are words that add information about a noun or tell the relationship between different things. A relative pronoun is a word, such as : who, whom, that or which that is used to introduce a relative clause. These words are called relative pronouns. The compounds whoever, whomever and whichever are also relative pronouns.

In other words, A pronoun which is used to begin a subordinate clause can be referred to as a relative pronoun, since it indicates the relationship of thee subordinate clause to the rest of the sentence. A subordinate clause which is introduced by a relative pronoun is often referred to as a relative clause .

Example : - The woman, who is wearing red saree, is a doctor. Here , who is a relative pronoun as it is used to refer to the people in the sentence .

In the question : -

1. The car that was stolen last month was found in a river. In this sentence , that is the correct relative pronoun as it is used in a defining clause that refers to a person, animal or thing .

2. Susan is the girl whom I was talking about. In this sentence , whom is the correct relative pronoun  as it refers to the object . Here, the noun Diya receives the action in the sentence which is whom i was talking about .

Thus , the Correct Option is D , which is that , whom .

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.

Relative Pronoun: - are words that add information about a noun or tell the relationship between different things. A relative pronoun is a word, such as : who, whom, that or which that is used to introduce a relative clause. These words are called relative pronouns. The compounds whoever, whomever and whichever are also relative pronouns.

In other words, A pronoun which is used to begin a subordinate clause can be referred to as a relative pronoun, since it indicates the relationship of thee subordinate clause to the rest of the sentence. A subordinate clause which is introduced by a relative pronoun is often referred to as a relative clause .

Example : - Paneer pasanda , which we eat at least twice a week, is one of my family's favorite meals. Here , which is a relative pronoun as it is used in a non-defining clause that refers to a person, animal or thing .

In the question : -

The person ________ stayed with me last night was my best friend. In this sentence , who is the correct relative pronoun as it refers to a person .

Thus , the Correct Option is D , which is who.

Pronoun : - A pronoun is a word that you use to refer to someone or something when you do not need to use a noun, often because the person or thing has been mentioned earlier. Examples are 'it', 'she', 'something', and 'myself'. '

Relative Pronoun:-  are words that add information about a noun or tell the relationship between different things. A relative pronoun is a word, such as : who, whom, that or which that is used to introduce a relative clause. These words are called relative pronouns. The compounds whoever, whomever and whichever are also relative pronouns.

In other words, A pronoun which is used to begin a subordinate clause can be referred to as a relative pronoun, since it indicates the relationship of thee subordinate clause to the rest of the sentence. A subordinate clause which is introduced by a relative pronoun is often referred to as a relative clause.

Example : - Sheela, who is a teacher, also works as a social worker. Here , who is a relative pronoun as it is used to refers to the person .

In the question : -

The car _______ was stolen last month was found in a river.

In this sentence , that is the correct relative pronoun as it is used in a defining clause that refers to a person, animal or thing .

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