Choose the most appropriate option to complete the given sentence:

Ashwini insisted that the committee must ____ the matter thoroughly before making any decision.

Option 1 -> "investigates" is incorrect because modal verbs like "must" are always followed by the base form of the verb, not the third-person singular present form.

Option 2 -> "investigating" is incorrect because modal verbs require the base form, not the gerund (-ing) form.

Option 3 -> "investigated" is incorrect because modal verbs cannot be followed by past tense forms.

Option 4 -> "investigate" is the base form of the verb, which is the correct form required after the modal verb "must."

Hence, Option 4: investigate -> Modal verbs (must, can, should, will, etc.) are always followed by the base form of the verb without "to." This is a fundamental rule of English grammar. -> correct