________ candidates left the hall before the viva had started.

In this question:
- "Candidates" is a plural countable noun.

- We know, countable nouns take many as their determiner, whereas uncountable nouns take much.
- On the other hand, few use for countable nousns whereas little is used for uncountable nouns.
- A few, A little, A bit is being used generally for uncountable nouns although a few is used for countable nouns as well.

So, by considering all and according to meaning, the best answer is , "several".

Source: British Council learning website.