Assuming Mutation Frequency to Be Around 10⁻⁶,at What Frequency Will

Assuming mutation frequency to be around 10⁻⁶,at what frequency will quadruple mutant auxotrophs revert to prototrophy?

A) 4 x 10-⁶
B) 12 x 10-⁶
C) 24 x 10-¹²
D) 1 x 10-²⁴
E) None is a correct choice