What compound-complex sentence can be formed
from the underlined section of the passageLearning a language can be a difficult and useful
hobby, I've tried it.
Learning a language can be difficult, it is one of the
most useful hobbies I've tried, Learning a language can be difficult, it is one of the
most useful hobbies I've tried,
Learning a language can be difficult, yet it is one of
the most useful hobbies that I've tried

The compound-complex sentence that can be formed from the underlined section of the passage (Learning a language can be a difficult and useful hobby, I've tried it) is the last one: "Learning a language can be difficult, yet it is one of the most useful hobbies that I've tried".

Explanation:

First, let's remember that a compound-complex sentence is a sentence made up of more than one sentence joined by a conjunction, and at least one of those sentences is complex. It is a compound sentence with a dependent, or subordinate clause. In this particular case, the two independent clauses are: "Learning a language can be difficult" and "it is one of the most useful hobbies". Together, they form a compound sentence joined by the conjunction "yet", and "that I've tried" is the dependent clause that makes the whole group a compound-complex sentence.