According to the law of diminishing marginal utility:

a. you should never consume more of something if your marginal utility is decreasing.
b. if your total utility is increasing as you consume more of something, then your marginal utility must be increasing as well.
c. as you consume less of something, your marginal utility from consuming that good will increase.
d. as you consume less of something, your total utility will decrease.

The correct answer is the letter b. Self interest.

Explanation:

According to Adam Smith “it is not the benevolence of the butcher, the brewer and the baker that we expect our dinner, but the consideration they have for their own interests” ie people starting their own business are acting in their own interest, so Adam Smith's idea that influenced Mark to start a business is self-interest. So Mark hopes to make a profit from his business so that he is primarily interested in his own earnings.

c.

clarify customers’ doubts only when asked.