Don’t run around. In the above, the word "Around" is a/an:

Don’t run around. In the above, the word "Around" is a/an:

Explanation

Several words are sometimes used as adverbs or sometimes used as prepositions.

A word is a preposition when it governs a noun or a pronoun, It is an adverb when it does not.

Don’t run around.  “Around” is an adverb here

Don’t loiter around the street. “Around” is a preposition here