In 2200 people ... to the Moon on holidays.

The sentence describes a future event that is expected to happen. The future simple tense 'will fly' is appropriate for expressing predictions or expectations about the future. The passive voice 'will be flown' would imply that people are being sent to the Moon by someone else, which is less likely in this context. 'Will be flying' is future continuous and suggests an ongoing action, which doesn't fit the context of a single event. 'Fly' is present tense and incorrect for a future event. Therefore, 'will fly' is the most suitable option to complete the sentence, indicating that people will travel to the Moon in the future.

Answer: will fly