Final answer:
The phrase "Let it go" is sung 21 times in the song. None of the provided options match this number exactly.
Step-by-step explanation:
The phrase "Let it go" appears 21 times in the song "Let It Go" from the movie Frozen. This includes the instances where the phrase is sung alone or as part of a longer line. None of the provided options (a) 6, (b) 12, (c) 18, or (d) 24 are correct. If we only count the distinct times the phrase is sung as a standalone line or part of a repeating chorus, the number is closer to the option (c) 18, but it still is not exact.