In a recursive solution to a problem, we solve a problem P(n) by solving another problem P(k) where

a. P(k) is the hardest part of P(n)
b. P(k) is a larger problem than P(n)
c. P(k) is smaller than P(n)
d. P(k) is the same problem as P(n) apart from size.

c), d)
Explanation: Recursion involves doing part of the task, P(n), leaving a smaller version of the same problem, P(k). There must be a stopping case, though this isn’t mentioned in this problem.