Question

Good morning, sir, what ____ I do for you? Choose the correct word to fill in the blank.
(a)would
(b)can
(c)shall
(d)must

Answer 1

Hint: Look for pointers suggesting which modal auxiliary to use in which blank. The sentence simply asks a question. These pointers are indicators of certain situations.

Complete answer:
Verbs are action words that show action (sing),
occurrence (develop), or state of being (exist), usually of a noun. Every sentence requires
a verb. The basic form of a verb is known as an infinitive.
A modal auxiliary is a type of verb that is used to indicate modality – that is: likelihood,
ability, permission, request, capacity, suggestions, order, obligation, or advice. Modal
verbs always accompany the base form of another verb having relevant linguistic content.

In the given sentence, the server is asking the person what to do. This indicates ability as the server should be able to do what the sir asks him to do. So, the modal auxiliary we are looking for should indicate ability.
 Now, we know what we are looking for. Let’s look at the given options:
(a)would - The modal auxiliary ‘would’ is used to indicate a past habit or a conditional verb. This modal verb does not indicate ability and so, it does not fit into the given sentence. This is not the required answer. So, this is an incorrect option.
(b)can - The modal auxiliary ‘can’ is used to indicate ability. This modal verb indicates the ability and so, it fits into the given sentence. This is the required answer. So, this is the correct option.
(c)shall - The modal auxiliary ‘shall’ is used to indicate future action. This modal verb does not indicate ability and so, it does not fit into the given sentence. This is not the required answer. So, this is an incorrect option.
(d)must - The modal auxiliary ‘must’ is used to indicate obligation. This modal verb does not indicate ability and so, it does not fit into the given sentence. This is not the required answer. So, this is an incorrect option.

Note: The modal auxiliary can is also used ‘to express permission (in the sense of being allowed or permitted to do something’ and ‘to express theoretical possibility’ for example, American automobile makers can make better cars if they think there's a profit in it.