Final answer:
The most effective sentence combination is 'Rachel met a singer, who lives in her apartment building.'
Step-by-step explanation:
The most effective sentence combination that uses correct punctuation is: Rachel met a singer, who lives in her apartment building. This sentence combines the two original sentences into one, and uses a comma to separate the main clause ('Rachel met a singer') from the relative clause ('who lives in her apartment building').
Learn more about Combining sentences with correct punctuation