I have no idea how A can be the right answer.
The argument in the passage seems to be:
- 5 senses traditionally viewed to be distinct yet complementary
- There are synesthesiacs, however, who mix their senses up
- Therefore, there are people whose senses do not respect traditional boundaries
The answer is that synesthesiacs are unable to speak or understand words. If this is true, then synesthesiacs cannot claim they are tasting blue, which would seem to undermine the argument. However, this is not necessarily so; even if they can't efficiently communicate that they are tasting blue, we know their senses are mixed up. If they're actually tasting green and are incapable of expressing that, it doesn't change the fact that they're tasting green.