This is more of a conceptual question than anything. Let's look at the implications of each statement individually:

a) If s and t are negative, then neither of them is zero. If the answer were in fact to be zero, then BOTH would have to be zero. If neither is zero, then it follows that both are not zero. We're left with A or D now as answer choices.

b) First, GRE will never try to take the root of a negative number (as far as this test is concerned, this is undefined), even if you might have seen that somewhere else. Stick to the rules of the exam, please . Second, both s and t are squared, so they will go positive in any case. There is no way that we will get the whole square-rooted term to go negative.

This CANNOT be negative so it is also a valid answer. We can see at this point that because A and B are both valid that D must be our answer.

c) Actually, the statement pretty clearly will be positive, so this answer is out.

d) This is the answer.

e) We've already shown A and B to match, so this answer is out.