A. To give more people a say in choosing candidates.
Explanation:
The Jacksonian years saw a call to transform candidate nomination from a closed caucus exercise to a more democratic procedure. And it was precisely in the era of Andrew Jacskon that party conventions started, as he defined and presented himself as the people´s president, the president of the common man. Also, the 1824 polemic and disputed election - "The Corrupt Bargain" - contributed to materializing the idea of party conventions. The practice started in 1832.
Pledging to vote for a candidate would be only positive for this candidate: so we have to choose an option that would not be a positive outcome for this candidate.
Such an option is "making the candidate appear weaker" and indeed, pledging to vote for a candidate would not make this candidate appear weaker. (answer B)
