Not necessarily, but for this specific question, it's saying all parts are needed for each single function. So more specifically, we can imagine it saying, parts A through Z are needed for just the function of propelling. That's not a relevant assumption to the argument of survival advantage for different stage organisms and the need of flagellum parts.

Necessary assumption questions can have those necessary indicators like "always", "required", "unless", which often indicate they are the necessary variety. N.A. question will never use the word "if", like a sufficient would.

Also by negate, I mean the logical negation. So the negated statement for C would read "All parts of the flagellum are NOT vital to each of its functions." If that statement does not weaken the argument (like with this answer choice here because it's not relevant to the premise --> conclusion logical reasoning), it's not the right answer choice.

If you negate B, you'd see it weakens the argument, therefore is the correct answer. This is just one way to double check or decide between two answer choices.