Good question! C is wrong because of the last part. "Have changed their position on free trade" is not a must be true, just a could be true.
It's possible that all of those politicians mentioned in the stimulus were never in favor of free trade with other Latin American countries, just in favor of free trade with Canada, the US, and Mexico. If that were the case, then it wouldn't be that they were changing their positions on free trade.
Additionally, the stimulus says that those politicians don't publicly support the idea. However, privately their positions on it may not have changed.