Answer:
The correct answers are: infinitive phrase; adjective.
Explanation:
The phrase we are looking at is to write about; given that it contains and infinitive, we can safely assume it's an infinitive phrase. Now, given that it modifies, or describes a noun, it means that it acts as if it were an adjective.
Since adjectives are words or phrases that modify nouns, the function of the infinitival phrase here is that of an adjective. It's not just any topic: it's the best topic to write about.
