The correct option is B The leading Democratic candidate was ex-President Martin van Buren, and the Whig candidate was Henry Clay.
The fact that Abraham Lincoln supported Whig Henry Clay and not the Democratic ex-President Martin van Buren is irrelevant to the context of the passage. For the following sentences to make sense, we only need to know about the two candidates for the post of the president. In this context, option B is the best answer. The other options are incorrect because they are inappropriate in the given context.