Answer: Infinitive Phrase: The best topic. Use: adjective.
Explanation: An infinitive phrase is a group of words that uses an infinitive (“to” + verb). It can function like a noun, adjective, or adverb in a sentence. In the given sentence we can see an example of an infinitive phrase in the words "to write about" in this case, it is used as an adjective (a word that modifies or describes a noun or a pronoun) because it is modifying the noun "topic."