He moved on. In the above sentence, the word "on" is a/an:

Adjective

Adverb

Preposition

None of the above

All of the above

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.

He moved on. “On” is an adverb here

Keep your books on the table. “On” is a preposition here