The correct option is
C had never flown
The given situation states a past event 'was very nervous...was his first flight'.Option C: 'Had never flown' is in past perfect tense. It states an action that completed in the past.
The past perfect tense indicates that an action happened 'had never flown' before another action took place 'it was his first flight'.
Before this flight, the man had never flown before.
Hence this option is relevant in meaning and C is correct.
Option A: 'Is never flying' is in present continuous tense. It states a continuous action in the present. But the situation has already happened in the past 'was his first flight'.
Option B: 'Has never flown' is in the present perfect tense. It states an action that just happened.
But, the action already happened in the past.
Option D: 'Was never flying' is in past continuous tense. It states a continuous action in the past tense. 'Never' means 'not ever'. It states an action that had not happened at all. Hence, we don't use the continuous tense verb 'flying'.
So, the verb form 'was + never + flying' isn't placed well.
Hence options A, B and D are incorrect.