Which of the sentences contains an adverb phrase? a which of the escalators go down? b they used the entrance door on the left. c my mother took the pictures on the digital camera. d you can come downstairs at midnight.

The answer here is B. The phrase is "on the left." It is telling you where. An adverb phrase can tell you either why, where, how, or when.

Since an adverb tells you either, when, where, why, or how, you should first look over all the options. 
Your answer would be 
B) They used the entrance door on the left.
This is the answer because it tells you where the location was and it was "on the left."
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