Hi Kmikaeli,
Good question! The opposite answer is one of the common incorrect answer choices in the must be true questions. Basically, rather than the answer choice being true, the stimulus proves that answer choice FALSE. So to determine if the answer choice is an opposite answer, check to make sure that the answer choice is proven TRUE rather than FALSE by the stimulus.
For a conditional statement: consider the following example (which is a bit simplistic but illustrates the point).
A
B
B
C
The correct answer would likely be A
C. But imagine we had an answer choice A
C. That would be a opposite answer--the stimulus proves that answer choice FALSE.
So I would agree that a mistaken negation could be an opposite answer for sure but I am not sure that I would say that it is limited to just mistaken negations.
At the end of the day, just make sure that you are selecting the answer choice that MUST BE TRUE. The label of the incorrect answers is not as important as understanding the concept behind it.
I hope that helps!