Answer:
none
Explanation:
The right answer is: Antarctica has a thick ice cover since it has the highest average elevation of all the continents.
A is wrong because the subordinating conjunction is introducing a dependent clause and there is an independent clause missing.
B is wrong because you do not use commas to introduce subordinating conjunctions. (it is the closest to the right answer though)
C is wrong because you never separate two independent clauses with a comma.
D is wrong for the same reason in A.
