Answer:
D: {0,1,2,3,4...}
Explanation:
c(x) gives her sales revenue, where x is the number of candles sold.
0
x
10 is not appropriate as she could sell more than 10 candles
All real numbers is not appropriate as she cannot sell a negative (or non-whole) amount of candles.
{-50,-40,-30,-20...} is not appropriate because again she cannot sell negative candles.
Therefore the last option is correct as she can only sell candles in positive integer amounts (or 0).