Question

(the candidates ______answer five out of ten questions) fill modal auxiliaries​

Answer 1

Explanation:

the candidates must have to answer five out of ten questions.

Answer 2

Should

Modal verb

A modal verb is a type of verb that contextually indicates a modality such as a likelihood, ability, permission, request, capacity, suggestion, order, obligation, or advice.

We will use "should", as it is used for saying that somebody had the ability or was allowed to do something. Now the sentence will be: the candidates should answer five out of ten questions.