The answer is B because when it asks for constant that means that it's always changing, the number of homes runs for answer A isn't constant with the amount of time in a game because even though the game lasts for 2 hours you could manage to make 2 home runs in the first 30 minutes, and in the rest of the game you don't make any at all. This causes the game to not always be constant in the rate of change with the time in the game. For c the answer is still not constant because of the same reason A wasn't the answer, because consistency isn't guaranteed. The same thing goes with D, while the puppy might begin to grow at a constant rate of change, once it's full grown the puppy will stop growing to leave the constant rate of change no longer viable. However with B, it says that the football was thrown in the air, they are measuring the height of the football in the air. The height will always have a constant rate of change as long as the football is in the air. Which the answer implies is the only way they are measuring the football's height.
Therefore, your answer is B.