Answer:
a) a horizontal restraint violating the Sherman Act
Explanation:
The Sherman Act of 1890 prohibited those activities that are restricted to the interstate and the competition arises in the market place
Here in the given situation, it is mentioned that the due to enrollment shrinking, it made the agreement for all the incoming students to give the free tuition for one semester
So this represents the horizontal restraint that consist of agreement within the same type of business with the motive of decreasing the competition