In this sentence, we have 2 independent clauses in the non-underlined portions:
1. "Mauritius was a British colony"
2. "the English language was never really spoken"

In the original sentence, these clauses are joined only with a comma. This is what we call a comma splice or run-on sentence.

In order to correctly connect 2 independent clauses, we must have one of the following:
1. a conjunction (e.g. and, or, but)
2. a semicolon
3. a coordinating conjunction (i.e. something that makes the 2nd clause dependent, e.g. "though" or "despite", etc)


Mauritius was a British colony for almost 200 years, excepting for the domains of administration and teaching, the English language was never really spoken on the island.

(A) excepting for
- comma splice
- the correct preposition is "in" --> we speak English *in* certain domains

(B) except in
- comma splice

(C) but except in
- correct!

(D) but excepting for
- this fixes the comma splice, but is not idiomatic

(E) with the exception of
- comma splice

The correct answer is C.