What most people do doesn’t affect the reasoning, which is based on specific aspects of this flower delivery to Drew. Someone who knew Drew would know Drew prefers violets. What matters is what people would do knowing that preference. What people would do in other circumstances doesn't matter.

This questions the assumption that someone who knew Drew prefers violets to roses would have sent violets. It’s possible someone who knew Drew well sent him roses, despite knowing his preference. This is how it’s possible the florist didn’t make a mistake.

This answer, if it does anything, would only suggest the flowers weren't sent by someone who didn't know Drew well. The author wasn't suggesting that the delivery had to come from such person, so (C) is consistent with the author's reasoning.

This doesn’t affect the reasoning of the argument, which was based on specific aspects of the flower delivery to Drew. The argument wasn’t citing to the florist’s past mistakes or general tendencies; an answer about the florist’s past doesn’t engage with the author’s reasoning.

The fact some people who know him well have sent him cards before, if it does anything, could only suggest another reason to think there was a mistake. Someone who knew him well could have sent a card, but he didn't get a card.