This weakens the argument because it introduces a significant difference between natural and synthetic pesticides. Humans haven’t had millennia to adapt to synthetic pesticides, so these synthetic pesticides could be harmful even though we regularly eat natural pesticides.

(B) weakens the argument because it introduces a difference in the way that natural and synthetic pesticides are used. It could be that pesticides become dangerous at a certain concentration, so the higher concentration of pesticides from synthetic pesticide use could cause harm.

(C) introduces another point of distinction between natural and synthetic pesticides––if the toxicity in synthetic pesticides is highly concentrated, then it makes sense that the synthetic pesticides could be harmful while natural pesticides aren’t harmful.

This is another difference between natural and synthetic pesticides––natural pesticides are targeted against specific parasites, so they wouldn’t damage humans. If synthetic pesticides are more broadly harmful, they could be harmful to humans.

The argument based on assumed similarities between natural and synthetic pesticides, so to weaken the argument, we need to demonstrate some distinction between the two types of pesticides that could mean that natural pesticides are safe, while synthetic pesticides aren’t.