In which of the sentences below is the semicolon used correctly to join two independent clauses?

a. Sammy fell into a deep sleep; he woke feeling refreshed and renewed.
b. After Sammy fell into a deep sleep; he woke feeling refreshed and renewed.
c. Sammy fell into a deep sleep; and he woke feeling refreshed and renewed.

a. Sammy fell into a deep sleep; he woke feeling refreshed and renewed.

The correct usage of a semicolon to join two independent clauses is found in sentence a. "Sammy fell into a deep sleep; he woke feeling refreshed and renewed."