Answer:
Infinitive phrase as an adjective.
Step-by-step explanation:
An Infinitive phrase is a group of words that is used in an infinitive way in a sentence as an adjective, noun, and adverb.
When an infinitive phrase is used as an adjective in a sentence they help to modify the nouns.
In the given sentence, the infinitive phrase is working as an adjective as it is modifying the noun, topic.
So, the correct answer is infinitive phrase as an adjective.