Answer:
The correct answer is option A "infinitive"
Step-by-step explanation:
The infinitive begins with the preposition "to" followed by the bare verb, in this case, "learn".
The infinitive can have different functions within a sentence. In this case, the infinitive "to learn" is following a common pattern before an adjective (difficult to learn). This common pattern is formed by verb to be+adjective+ infinitive: "The Chinese language is difficult to learn"
The rest of the options are incorrect since an appositive phrase is a noun phrase that provides additional information about another noun.
"To learn" is not the verb as it not conjugated. The main verb in the sentence is "is".