As I am on this site to help, I will assist in your understanding. However, please don't make this a regular occurrence. FLVS quizzes are supposed to be taken without the help of others.
Anyway....
A domain of all real numbers means that a function will have an infinite amount of x-values. As g(x) and f(x) both are line segments, p(x) is the only function with a domain of all real numbers.