Help the quirky inventor finalize the safety manual for his time machine by choosing the correct word.

____ you accidentally step on a Jurassic mosquito, how would you prevent a catastrophic alteration of the timeline?

The correct answer is Supposing.

"Supposing" (or "supposing that") is used as a conditional conjunction to invite someone to imagine a hypothetical situation, often at the beginning of a question. "Provided that" and "As long as" mean "only if" and do not fit the context of a hypothetical question, while "Unless" would illogically mean "If you do not step on a mosquito."