Which of the Following Is(are) Required Condition(s) for a Discrete $\Sigma$

Multiple Choice

Which of the following is(are) required condition(s) for a discrete probability function?

A) $\Sigma$ f(x) = 0
B) f(x) $\ge$ 1 for all values of x
C) f(x) < 0
D) None of the answers is correct.