Answer:
This is a simile.
Step-by-step explanation:
A simile is a comparison in a sentence where the words 'like' or 'as' are used. In the sentence, "as sharp as a knife," whatever the object is, is being compared to a knife because of how sharp it is. The key indicator or the 'keyword' is "as" as that immediately indicates that the sentence is a figure of speech, which is as mentioned before, a simile.