Not necessary, because the author merely believes that the parts would not provide a survival advantage. That doesn’t imply the author thinks the parts would be harmful to an organism that possessed them. They might have had no effect, and so put an organism with the parts on equal footing compared to an organism without the parts.

Necessary, because if it were not true — if the parts could have aided an organism’s survival even if they did NOT help it swim — then we cannot conclude that those parts provide no survival advantage merely because they didn’t help the bacteria ancestor swim. (B) is the assumed link from the premise to the conclusion.

The reasoning is based on the fact that many parts are required for the flagellum to help a bacteria swim; the specific importance of different individual parts isn’t relevant to that reasoning. Also, the argument concerns only the flagellum’s function in helping a bacteria swim; other functions aren’t relevant.

Not necessary, because there could have been some ancestors that had only a few parts of the flagellum. The author would then believe that for those ancestors, the few flagellum parts did not aid in swimming and therefore provided no survival advantage.

Not necessary, because the argument concerns only the survival advantage of the few flagellum parts. The author thinks because having these few parts wouldn’t help the bacteria swim, these few parts didn’t provide a survival advantage. But if the bacteria could swim through other methods unrelated to a flagellum, that doesn’t relate to whether the few flagellum parts were helpful for swimming or provided a survival advantage.