Johnson attacked his opponent by taking her quote out of context. Johnson's campaign defended this action by saying that the quote out of context is less politically damaging to her than the quote in context would be. In other words, they're saying that Johnson took the route of less harm by paraphrasing the quote in a way that made it seem better than it actually was, so he should not be blamed. However, the analyst concludes the campaign managers do not actually believe their own logic because they could have shown the quote in context to prove it is worse, but they continue not to do so.
(A) "In criticizing an opponent, political campaigns will pursue the line of attack they believe to be most politically damaging." This supports the analyst's claim that Johnson's campaign managers do not truly believe that Johnson took the route of less harm. If they did believe this, they would have referred to the quote in context, but they continue not to do so. Therefore, they are pursuing what they believe to be the most damaging line of attack- quoting Johnson's opponent out of context. So, (A) supports the analyst's reasoning and is correct.
Does that make sense? Hope it helps! Please let us know if you have any other questions!