A constant is a number that stays the same no matter what any outside variables are. In this case, the variables are the number of hours of the event and the number of attendees. However, it states in the problem that the event will be 5 hours so that becomes known; the only thing that might change is the number of attendees so you can eliminate any cost that will be affected by that. You are left with Facility Rental, Musical Entertainment, and Cleaning Fee. Furthermore, you can be sure that Musical Entertainment and Cleaning Fee must be included in the constant because they are described as "flat rates", which means they do not change.
The answer is D) It is the total cost for entertainment, facility rental, and cleaning.
Hope this helps!