Solution
= Solution
{c}
For $\mathbb P(X=x)>0$, the <conditional probability mass function> is
$$
\boxed{p_{Y\mid X=x}(y)=\frac{\mathbb P(X=x,Y=y)}{\mathbb P(X=x)}.}
$$
The denominator is the sum of the numerator over all possible $y$, so these <conditional probabilities> sum to one.