Fill in the blank with the most appropriate modal auxiliary :
Your mother is seriously ill. You ______ leave for home at once.
(obligation)

Question

Accepted Answer

The correct option is (4):
The most appropriate modal auxiliary to fill the blank is "must." The context provided is "Your mother is seriously ill," which establishes a severe and urgent situation. Following this, the word "(obligation)" explicitly tells us that the modal must convey a strong sense of necessity or duty. The modal auxiliary "must" is specifically used to express a strong, often urgent, necessity or obligation arising from the speaker’s own conviction or from the seriousness of the situation. In this case, the illness of the mother makes leaving for home "at once" an absolute and unavoidable duty, perfectly matched by the strength of "must."

The incorrect options are (1), (2), and (3):

The incorrect option is (1):
"Needs" is not a modal auxiliary verb in this context; it functions as a lexical verb, and while it indicates necessity, it is grammatically awkward here without a preceding auxiliary verb (e.g., "You need to leave"). Modal auxiliaries like "must" are used directly with the base form of the main verb ("leave") without "to." Furthermore, "must" conveys a stronger sense of immediate, non-negotiable obligation than "needs."

The incorrect option is (2):
"Might" is used to express possibility or weak probability (e.g., "It might rain later"). It suggests that leaving is possible but not certain or necessary. This meaning directly contradicts the instruction to convey a feeling of strong *obligation* and the seriousness of the mother's illness.

The incorrect option is (3):
"Could" is the past tense of "can" and is typically used to express past ability (e.g., "I could run faster when I was younger") or polite possibility/request. Although it can express a slight possibility, it does not convey the high degree of strong, present obligation required by the sentence, which involves an immediate and critical family situation.