Explanation:

Correct answer is (b). According to the given sentence, it is stating an event standing at a point when something has continued for some time and now some decision is to be taken. Hence, at the point of reference, the action has continued for quite some time and is in progress at present as well. This is described by the Present Perfect Continuous tense which takes ‘have/had + been + ‘-ing’ form of the verb’.
Secondly, 'under the sun' is an idiomatic expression which means 'in existence (used in expressions emphasizing the large number of something)'.
Ex. They exchanged views on every subject under the sun. Therefore, 'in the sun' and not 'under the sun' is the correct here in the given context of the sentence.
The correct sentence will therefore be: The contestants have been waiting in the sun for more than six hours now, so, it is unfair to cancel the whole audition because of the absence of one crew member. This makes Option B the right choice.