Fill in the blank with a suitable verb:
He __________ drive a car when he was eighteen.
(a)is able
(b)could
(c)can
(d)is used to

Hint: A modal auxiliary is an auxiliary verb (such as `can' or `will') that is used to express modality. Modality refers to a classification of propositions on the basis of whether they claim necessity or possibility or impossibility.

Complete answer:In the given question, we have to find out which of the options are in past tense (as he was able to drive a car when he was eighteen).
Let us thus analyze the options given to us in this question:
(a)'is able', refers to a phrase that is grammatically incorrect as ‘is’ is a preposition which is generally used after an adjective like ‘able’.
Therefore, option (a) is incorrect.
(b)‘could', is used to express an ability in the past, for example : “I could always beat you at tennis when we were kids.” The above sentence, talks about a time when the said person ‘was eighteen’ and ‘was able to drive at that point of time’,referring to the past. So, Option (b) is the correct answer.
(c)‘can',is used to express ability (in the sense of being able to do something or knowing how to do something), for example: ‘He can speak Spanish but he can't write it very well. Therefore, option (c) is incorrect.
(d)‘is used to', refers to a phrase that is grammatically incorrect.
Therefore, option (d) is incorrect as a grammatically incorrect option cannot be the answer.

Note: In this question, it will be helpful for the student to recall the concept of modals. Many modal verbs cannot be used in all of the English tenses,that's why students need to know the substitutes to these modal verbs.