You should not have posted that option A was incorrect. i almost chose that option
So then, after further review i stopped on B,
let me try to explain why A is not the answer
The choice claims that "If the political platform of the third party is a compromise position between that of the two major parties, the third party will draw its voters equally from the two major parties."
But if you look to the text of the question, it is very cler that the third party candidate always takes votes either from one of candidates but not both. Thus it means, third candidate does not take equal vote from both parties, but takes just from either of one. A claims it took equal votes from both candidates, which means it is wrong.
Hope it helps.
Never give up,,,