#### Answer

a.) continuous
b.) discrete
c.) discrete
d.) continuous

#### Work Step by Step

A discrete variable is a variable that can only take on a finite number of values in an interval. The number of courses a student is taking is discrete because it can only be an integer, just like the number of cents in your pocket. 3.4 courses make no sense, and you cannot carry money that's value less than a cent.
On the other hand, continuous variables can take on infinite number of values. The playing time of a CD is continuous just like a distance, because you can be infinitely precise and write infinitely many decimal places, (5.4618744162315... minutes)