Which of the following is not an option for joining two independent clauses to make a compound sentence?

comma + coordinating conjunction

comma

semicolon + conjunctive adverb + comma. semicolon

The correct answer is comma.

You cannot join two independent clauses in order to make a compound sentence just by using a comma. If you only use a comma (without any conjunctions), then this is going to be a run-on sentence.

For example:

I saw a boy at school, I think he is the most beautiful boy in the world.

Because you didn't use a conjunction (and) or a semicolon, this sentence is compound, but it is incorrect.