What part of speech is the highlighted word in each sentence?

Choose the correct option.

Oliver likes to read books in his free time. [Noun|Verb|Adjective|Adverb]

I enjoy listening to music while I do homework. **** [Noun|Verb|Adjective|Adverb]

They are learning a new language. [Noun|Verb|Adjective|Adverb]

My father often goes to Japan. [Noun|Verb|Adjective|Adverb]