This is necessary for the conclusion. If negated, this would destroy the argument: what if three months isn’t enough time to see a significant change, but the drugs are still helping?

This is also a sufficient assumption.

This weakens the argument. We know that there has been a change in average vertigo duration since this drug has been introduced, so (B) would let us infer that the drug has an effect on vertigo—the opposite of our conclusion.

If negated, this does not wreck the argument. It’s fine if more time would have been better; the argument can survive that as long as three months is enough to judge whether the drug has an effect.

The negation of (C) would strengthen our argument, and that’s the opposite effect that we want from a negation. Like (B), this weakens our argument, because it eliminates a possible alternative cause for the change in average vertigo duration.

We do not need drugs to be the only factor involved in decreasing vertigo duration. The argument is concluding that a specific drug is not decreasing vertigo duration, and this could be true regardless of how many factors there are.