A domain is defined as the set of possible values of a function which will give the output value. It is the set of possible values for the independent variables.
M(n) is the amount of money Trudy will raise while n is the number of full laps Trudy completes.
Thus, the domain is;

The domain is all integers from 0 to 30, inclusive.
The domain represents the number of full laps Trudy completes.