A normal spontaneous mutation rate for a single gene is one mutation in every ............ replication.

The correct option is C 10${}^6$- 10 ${}^8$
Spontaneous mutations are the type of mutations which occur during DNA replication, the rate of spontaneous mutation for a single gene is 10^6 to 10^8 bases in every replication.

So, the correct answer is ' ${10}^6-{10}^8$'.