Choose the most appropriate option to complete the given sentence:-

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

investigate

The correct answer is Option (4) → investigate

The modal verb must requires the base form of the verb (the infinitive without "to") to follow it.

• Must investigate is the correct structure.

• Investigates (Simple Present, third person singular), investigating (Present Participle), and investigated (Simple Past or Past Participle) are all grammatically incorrect after the modal auxiliary verb must.