The number of interceptions made by a defensive back is definitely countable because it is finite and also it is not continuous because each interception is different and is unique. That is to say, one interruption happens and then then next. They do not happen in a continuous manner.
Thus, the number of interceptions made by a defensive back are countable and are discrete. Having a look at the options we can see that Option C matches as it says "the variable is discrete because it is countable".
Thus Option C is the answer.