The cost C of renting a computer is a function of the number of days d that Hamlet rents the computer, where C(d) = 88d.
The domain of a function is the set of all possible input values for the function. In this case, the number of days Hamlet rents the computer cannot be negative, since he cannot rent it for a negative number of days.
Therefore, the domain of the function is all non-negative integers, or the set {0, 1, 2, 3, ...}.
So, the answer is C. All natural numbers.