Final answer:
The correct answer to the riddle is 'honesty,' signifying that the true nature of honesty is revealed through action. Franklin did not count mercy among his thirteen virtues. Poverty can sometimes pressure honest individuals into dishonesty to survive.
Step-by-step explanation:
The riddle you've asked about is: "_______'s plain face is never seen till used." The correct answer to this riddle is honesty. This saying points out that honesty is not always apparent until it is demonstrated through actions. For example, one can claim to be honest, but it is through honest actions and truth-telling that their honesty truly shows, much like a plain face that goes unnoticed until it is put to use.
Regarding the virtues of Benjamin Franklin, mercy was not originally listed as one of his thirteen virtues which included sincerity, temperance, and tranquility, among others. The statement "No one who knows that the result of honesty is always positive would choose wickedness when s/he is aware that it has a negative reward" emphasizes the value and positive outcome of being honest versus engaging in knavery.
In the context of poverty and its temptations, honesty can sometimes be seen as a luxury that the poor cannot afford, contributing to the impression that desperate circumstances can turn honest people towards dishonest deeds in order to survive.