Question

if warn means \to inform someone of a problem,\ then forewarn means to
a. ask someone why there are no problems
b. find a solution to a current problem
c. inform someone of a future problem
d. remember a past problem

Explanation:

The prefix "fore -" often means "before" or "in advance". Given that "warn" is to inform of a problem, "forewarn" is to inform of a problem in advance, i.e., a future problem.

Answer:

C. inform someone of a future problem