I will phone Sushma before she starts her onward journey. Which category does the underlined word belong to?

Verb

The correct answer is Option (2) → Verb

The underlined word  "PHONE" is a Verb In the given sentence.

Verbs describe actions, states of being, or occurrences. In this case, "phone" describes the action of making a call.
Nouns are words that name people, places, things, or ideas. (e.g., Sushma)
Prepositions show the relationship between a noun or pronoun and another word in the sentence. (e.g., before)  

Adjectives describe nouns or pronouns. (e.g., onward journey)