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