Which of the Following Is a Required Condition for a Discrete

Which of the following is a required condition for a discrete probability function?

A) f(x) $\leq$ 0 for all values of x
B) $\Sigma$ f(x) = 1 for all values of x
C) $\Sigma$ f(x) = 0 for all values of x
D) $\Sigma$ f(x) $\geq$ 1 for all values of x