I think the correct answer to the question being ask is A.
C) When traveling in space, astronauts weigh less because of a lack of gravity.
The best answer here is choice B. We can determine this because of the tense used throughout the passage itself. Bearing this in mind, choice B matches the tense perfectly.
Choice A does not work at all because the verb "am" makes no sense in the context of the sentence. The same is true for choice C. The passage is not in present tense, so it cannot be this choice. Choice D could make sense, except that this is in the future tense versus the past tense.