Multiple choice The alarm system of a car must ........ reliably. function to function functioned functioning Reveal answer Fill a bubble to check yourself A Correct answer Explanation After modal verbs like 'must', we use the base form of the verb without 'to'. The sentence is saying the alarm system has an obligation to work reliably. 'Function' is the correct base form, not 'to function', 'functioned', or 'functioning'.