If P(X=k)=k/6P(X=k) = k/6P(X=k)=k/6 for k∈{1,2,3}k \in \{1, 2, 3\}k∈{1,2,3}, is this a valid PMF?
Yes, because the probabilities sum to 1.
No, the probabilities sum to more than 1.
No, some probabilities are negative.
Yes, because kkk is positive.