Answer:
To fill in the blank correctly, we need to identify the correct form of the verb.

In this sentence, the action of filling in a form for a new cheque book comes before the action of using up all the cheques in the old book. This suggests that the action of filling in the form will happen in the future.

The verb "have" can be used to express future actions. In this case, we use the present tense form "has" to indicate that the action will happen in the future.

Therefore, the correct answer is option 'A': "Has."

Explanation:
The sentence is in the present tense because it talks about a current situation - the need to fill in a form for a new cheque book. However, the action of filling in the form will happen in the future.

The present tense form of the verb "have" is "has," and it is used to indicate future actions. It shows that the subject (he) will have to perform the action of filling in the form.

Here is the breakdown of the options:

a) "Has" is the present tense form of the verb "have." It is used to indicate future actions. This is the correct answer.

b) "Will have" is the future tense form of the verb "have." It suggests that the action of filling in the form will happen in the future. However, the sentence is already in the present tense, so using the future tense is not necessary.

c) "Had" is the past tense form of the verb "have." It indicates that the action of filling in the form has already happened in the past. This is not the correct form to use in this sentence.

d) "Should have" is a modal verb construction that indicates a missed obligation or expectation in the past. It suggests that the action of filling in the form was expected to happen in the past, but it did not happen. This is not the correct form to use in this sentence.

To summarize, the correct form of the verb to fill in the blank is "has" (option 'A'). This form indicates that the action of filling in the form for a new cheque book will happen in the future.