An adverb phrase is a group of words that does the work of an adverb, that is, modify the verb. Thus, the adverb phrase can be rewritten with an adverb.
Option D is correct. The adverb phrase "at that moment" has been replaced by an adverb "then" that not only modifies the verb "arrived", but also retains the meaning of the original sentence. "At that moment" can be rewritten as "then".
Option A is incorrect. While "now" is an adverb that replaces the adverb phrase. it does not mean the same as "at that moment". "Now" is a replacement of the adverb phrase "at this moment".
Option B is incorrect for the same reason as A. The adverb "there" does not mean the same thing as "at that moment". The former refers to place while the latter refers to the time of the action.
Option C is incorrect for the same reason as A. While "late" is an adverb, it does not mean the same as "at that moment". The phrase refers to a specific point in time while "late" does not.