If a Thin Soap Film N = 1

If a thin soap film n = 1.36) reflects predominately red light about 680 nm) , then what is the minimum thickness of the soap film?

A) 1.25 × 10^-7 m
B) 6.80 × 10^-7 m
C) 2.50 × 10^-7 m
D) 5.00 × 10^-7 m
E) None of these is correct.