Final answer:
The claim that people are mostly honest is best supported by Feldman’s success with his honor-based payment system.
Step-by-step explanation:
The statement that best strengthens Feldman's claim that people are mostly honest is 'Feldman’s payment system was largely successful.' This directly demonstrates the effectiveness of his honor-system commerce and the trustworthiness of his clients. Feldman's method relied on people being honorable enough to pay for what they took without supervision. The fact that he was delivering 8,400 bagels a week to 140 companies and made as much money as his previous job as a research analyst corroborates the success of his honor system and suggests that people were indeed mostly honest, as they were paying for their bagels despite the absence of oversight.