A business analyst uses a function, Q, to analyze vending machine revenue. Q (n) represents the revenue from the machine, where n is the number of quarters the machine received over a specified time period. Which of the following would be a possible domain?

A possible domain could be (0,1,2,3 ...)

If n is the number (not the value) of quarters received, then n cannot take values which are fractions and n cannot take negative values. Its values have to be non-negative integers. (Note that 0 is possible.)

That eliminates the descriptions in answers a, b, and c, leaving only answer

d (0,1,2,3, ...)

as a possibility. Since it describes the non-negative integers, it's the correct one.