Final answer:
Franklin prioritized Temperance as the first step toward moral perfection due to its importance in fostering personal discipline and civic responsibility, aligning with the ideals of the Temperance Movement widely supported during the 1820s-1840s.
Step-by-step explanation:
Benjamin Franklin began his journey toward moral perfection with the virtue of Temperance because he believed it to be foundational to the cultivation of other virtues. Temperance, which calls for moderation or self-restraint, especially in alcohol consumption, was seen as an essential step in improving one's own moral character and by extension, the moral fabric of society. In the context of the 1820s-1840s, the Temperance Movement was widely supported by reformers who linked intemperance with a host of social ills. Religious leaders, the middle class, and later, organizations like the Washington Temperance Society saw temperance as a keystone for a sober, industrious, and orderly society. Franklin's prioritization of temperance likely stemmed from a similar understanding of its importance in fostering personal discipline and civic responsibility, which in turn, supported his broader aspirations for moral perfection.
It should be noted that option C, 'mercy,' is not one of Franklin's thirteen virtues according to the list provided in the question. Franklin included virtues like sincerity, tranquility, and temperance among his thirteen virtues but not mercy.