Answer:
Compound-complex sentence can be formed from the underlined section of the passage is Option D: Learning a language can be difficult, yet it is one of the most useful hobbies that I've tried.
Explanation:
A compound-complex sentence is formed by joining two or more sentences which are joined by using a conjunction. It is formed by two 'independent' clauses and one or more 'dependent' clauses joined with a conjunction. Option D has two independent clauses "Learning a language can be difficult" and "I've tried". The dependent clause is "it is one of the most useful hobbies". They are joined using the conjunction 'yet'.
Other Options are not using appropriate conjunction to join the sentences correctly and form a 'compound-complex' sentence.
