The domain is all possible x- values. Since the graph starts at 0 and the line ends at 6 the answer must be (A) real numbers from 0 to 6. The following are reasons why the other answers are incorrect. (B) would allow x to be negative which is not allowed on this graph. (C) would allow x to be greater than 6 which is where the line ends. (D) would allow negative numbers.
