The man sitting next to me on the plane was nervous because he _ before.

Explanation

The use of "was" in the first part of the sentence and the word "before" at the end suggest it to be a past sentence. So we can rule out the second last option as it uses present perfect tense. The last option is incorrect itself as "been" is only used with perfect continuous tense. It will be wrong to use " has been flown", the correct structure for this will be, "has been flying". Simple past tense is not a very suitable option as well. The correct use of tense here will be the past perfect tense. Past perfect tense states an action that happened ( had never flown ) before another action took place ( his first flight ). So this implies that he had not flown before his first flight. Now the complete sentence will be as follows:"The man sitting next to me on the plane was nervous because he had not flown before".