I feel like there are just as many assumptions, if not more, that you need in order to accept B as correct as for C. For B, first you need to assume that nanobes are single-celled creatures. If they are not, then what single-celled creatures can or can't do is irrelevant. Then, you need to assume that the single-celled creatures are small, because if they are not, then their ability to reproduce has no bearing on whether nanobes might be able to reproduce, since the stem explicitly states that the reason nanobes can't reproduce is because they are too small. Even if you assume both of these, you still need to also assume that nanobes fall into the same subset of single-celled creatures as the ones that can reproduce in this unusual way in order to get to the idea that nanobes might be able to reproduce and therefore qualify as living things.

For C, yes you do need to assume that the scientists' claims are true or at least possibly true. However, if you assume that, then this answer directly weakens the core premise of the argument -- that nanobes are too small to contain a reproductive mechanism. We know that all bacteria contain reproductive mechanisms because all bacteria are alive. That's basic biology. So if something just as small as a nanobe has the capacity for a reproductive mechanism, then there is no longer any reason to believe that nanobes cannot contain a reproductive mechanism because they are too small.