Final answer:
Mercy was not one of Benjamin Franklin's thirteen virtues; sincerity, temperance, and tranquility were included in his list.
Step-by-step explanation:
The question pertains to Benjamin Franklin's thirteen virtues, which he developed as a moral code to live by. Among the virtues he listed were sincerity, temperance, and tranquility. Reviewing the list of Franklin's virtues reveals that mercy is not included as one of them. Therefore, the answer to the question 'Which of the following was not one of Franklin's thirteen virtues?' is C. mercy.