A: When it’s dribbling!
Explanation: “To dribble” the basketball means to bounce the ball as you walk or run down the basketball court. “To dribble” is also to let saliva drop from your mouth. Babies often dribble, that is, they drool, they let saliva or food or milk come out of their mouths.
This joke is funny because it plays on the double meaning of the word dribble; babies cannot dribble a basketball but they dribble all frequently.