The equation given is:
396 = 66t
This equation relates the distance traveled (396 miles) to the rate of travel (66 miles per hour) and the time it takes to travel that distance (t hours).
We can rearrange this equation to solve for the constant of proportionality, which is the rate of travel:
66t = 396
Dividing both sides by 66, we get:
t = 6
Therefore, the constant of proportionality in the equation is 66. The correct answer is C.