I think C but I'm not sure maybe it A
Answer:
The answer to your question would be that the revision that combines the sentence using a subordinating clause and subordinating conjunction is the following one: East Antarctica is covered by massive layers of ice which is thousands of meters thick.
Explanation:
The revised version of the sentence includes an essential relative clause. Essential relative clauses limit an ambiguous noun, that is, they complete the meaning of the sentence. The essential clause helps identify the noun it modifies. Consequently, if they are dropped, the meaning of the sentence is altered (see 1).
1) East Antarctica is covered by massive layers of ice
