Fill in the blank with the most appropriate word:
I could tell you the time if I ______ a watch.
A. Had had
B. Had
C. Would have
D. Will have

Hint: An auxiliary verb (helping verb) is a verb that adds functional or grammatical meaning to the clause. Auxiliary verbs usually accompany a main verb. For example: has, had, being, been, will, would, shall and etc.

Complete step by step answer:
Option A) Had had – is an incorrect answer because ‘had had’ is a past perfect form of have (had + past participle form of have). Had had is used when we are talking about the past and want to refer back to an earlier past time. So, we cannot use this in our sentence. Thus, this is an incorrect answer.
Option B) Had – is the correct answer because it is a conditional clause because the impact of the main clause of the sentence is conditional on the dependent clause. Thus, this is the correct answer.
Option C) Would have – is an incorrect answer because ‘would have’ is also a conditional clause and we use ‘would have’ to talk about something that did not happen in the past. Thus, this is an incorrect answer.
Option D) Will have – is an incorrect answer because the sentence is in present tense and ‘will have’ is used when there is any activity that is about to happen in future. Thus, this is an incorrect answer.
I could tell you the time if I had a watch.

So, the correct answer is “Option B”.

Note:
In such questions we have to understand the meaning of the helping verbs and also to understand the conditional clause. Basically, conditional clauses are the conditional sentences that express one thing contingent on something else. For example: If it rains, the picnic will be cancelled. They are the impact of the main clause on the dependent clause. The sentence is in present form so the answer should also be in the present form.