Modals: They are verbs which are used before the main verb to express ability, possibility, obligation, permission, suggestion, request, certainty or necessity.They indicate the attitude, intention and the mood of the speaker.

• They cannot be used alone in a sentence.
• In case of Questions they are used alone.
• They are not affected by the change in person i.e. first, second or third form.
• Present Participle or Past Participle form of a verb is never used with a modal.

Use of Was Able :  Be able to  is not a modal verb. It is simply the verb be plus on adjective (able) followed by the infinitive. We consider it as a modal because we sometimes use it instead of can and could. We use be able to, to talk about ability.

Structure of be able to: Subject + be + able + to-infinitive

Notice, that be able to is possible in all tenses, for example:

• I am able to write .

In the question : -

1. They ___________  to write correctly .

In this sentence , are not able is the correct modal that can be placed in the blank part as it is used to show an ability to do something  . Here , not is a negative form .

2. Will they _____________ to find the key ?

In this sentence , be able is the correct modal that can be placed in the blank part as it is used to show an ability to do something  .

3. Will she ____________  to cope with the work ?

In this sentence , be able is the correct modal that can be placed in the blank part as it is used to show an ability to do something  .

Thus , the Correct Option is C , which is 1. Are not able 2. Be able 3. Be able .

Modals: They are verbs which are used before the main verb to express ability, possibility, obligation, permission, suggestion, request, certainty or necessity.They indicate the attitude, intention and the mood of the speaker.

• They cannot be used alone in a sentence.
• In case of Questions they are used alone.
• They are not affected by the change in person i.e. first, second or third form.
• Present Participle or Past Participle form of a verb is never used with a modal.

Use of Was Able :  Be able to  is not a modal verb. It is simply the verb be plus on adjective (able) followed by the infinitive. We consider it as a modal because we sometimes use it instead of can and could. We use be able to, to talk about ability.

Structure of be able to: Subject + be + able + to-infinitive

Notice, that be able to is possible in all tenses, for example:

• I am able to write .

In the question : -

1. I ___________  to finish my test yesterday.

In this sentence , wasn't able is the correct modal that can be placed in the blank part as it is used to show an ability to do something in past . With singular subject , we use was . Here , not shows negative form .

2. They ____________  to catch six fish on their trip.

In this sentence , weren't able is the correct modal that can be placed in the blank part as it is used to show an ability to do something in past . With singular subject we use were . Here , not shows negative form .

Thus , the Correct Option is B , which is 1. Wasn't able 2. Weren't able .

Modals: They are verbs which are used before the main verb to express ability, possibility, obligation, permission, suggestion, request, certainty or necessity.They indicate the attitude, intention and the mood of the speaker.

• They cannot be used alone in a sentence.
• In case of Questions they are used alone.
• They are not affected by the change in person i.e. first, second or third form.
• Present Participle or Past Participle form of a verb is never used with a modal.

Use of Was Able :  Be able to  is not a modal verb. It is simply the verb be plus on adjective (able) followed by the infinitive. We consider it as a modal because we sometimes use it instead of can and could. We use be able to, to talk about ability.

Structure of be able to: Subject + be + able + to-infinitive

Notice, that be able to is possible in all tenses, for example:

• I am able to write .

In the question : -

1. I will ____________ to help you tomorrow, no problem ! I

n this sentence , be able is the correct modal that can be placed in the blank part as it is used to talk about ability in the future .

2. Since she is in home, she will _______________  to open the door for us .

In this sentence , be able is the correct modal that can be placed in the blank part as it is used to talk about ability in the future .

3. He will ____________  to drive himself once he gets his driver’s license .

In this sentence , be able is the correct modal that can be placed in the blank part as it is used to talk about ability in the future .

Thus , the Correct Option is D , which is 1. Be able 2. Be able 3. Be able .