The function used to estimate the revenue is:
R(x)= 79 x where R(x) is the revenue and x is the number of rooms booked.
Therefore, since the Domain of the function is the set of x-values that can be used to provide a valid result, that means that the set of possible number of rooms booked is the Domain.
That is any whole number between 0 and 220 (the maximum number of rooms that could be booked)
This coincides with the third answer listed.