This leads to the wrong conclusion. E and Z are both more harmful than T. But the doesn’t conclude that E and T should not both be legal or that Z and T should not both be legal. Instead, he concludes that E and Z should both be illegal, or else that T should be legal.

This is irrelevant because E and Z are both harmful to the environment. The conclusion allows for the possibility of E and Z remaining legal, even though they’re harmful. So (B) can’t describe the principle that helps justify his argument.

The author is comparing the harmfulness of pesticides. He says that if one pesticide is more harmful than another, the more harmful one shouldn’t be legal while the less harmful one is illegal. (C) says that if two pesticides should both be illegal, then they must both be harmful.

E/Z should be legal and T illegal only if E/Z are less harmful than T. In other words, if E/Z are legal while T is illegal, then E/Z must be less harmful than T. Since E/Z are more harmful than T, this supports the conclusion that E/Z should be illegal, or else that T should be legal.

This can’t describe a principle that justifies the argument, since none of the pesticides are described as harmless. The author doesn’t compare harmful pesticides to harmless ones; he compares more harmful pesticides to less harmful pesticides.