The correct answers is B,C, and D. It doesn't necessarily lead to an expanded vocabulary, but it certainly helps in all other aspects listed here.
The correct answer is actually 1. 2. 3. and 5.
Answer:
We stayed in a five-star hotel where Princess Diana once stayed.
Explanation:
To find the main clause in a sentence, look for a subject and a verb. Once you've identified these, see if the clause would make sense as a stand-alone sentence.