A - Correct
The proportionality constant is the number that associates the two different quantities. In this case, the cost is 5.5 times the number of tickets, therefore, it is the constant of proportionality between the cost and the number of students.
B- Incorrect
If you plug in x=1, you get the cost for exactly one ticket. In this case, C(1)=5.5, so the unit rate is 5.5
C- Correct
If you plug in x=2, you get the cost for two tickets. Therefore C(2) = 5.5x2;
C(2)= 11
So the cost of two tickets is $11.00