What compound-complex sentence can be formed from the underlined section of the passage?
Have you ever thought about traveling to another country? Pick a place that interests you, and where your native language is not spoken. Once you have picked a place, you can research language-learning options. You can use online programs or find a class. You could learn and practice with a friend, and then take a dream vacation. Learning a language can be difficult. It is one of the the most useful hobbies. I’ve tried it.

-Learning 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.

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 a sentence which consists of two (or even more) independent clauses and at least one dependent clause. These clauses are joined by a conjunction. In this case, the independent clauses are: 1) "Learning a language can be difficult" and 2) "it is one of the most useful hobbies". These clauses are joined by a conjunction "yet", and a dependent clause "that I've tried" is added, which makes a compound-complex sentence.

d is the best answer for this question.

the following key ideas correctly express the key sentence.

restate key idea: most people can learn to swim in ten short lessons.

restate key idea: the majority of water-accident victims require mouth-to-mouth breathing.

restate key idea: an empty, tightly closed gallon jug will support a tired swimmer.  

restate key idea: first aid is to prevent accidents as well as assist in rescue.

explanation: the key idea needs to have the same topic as the key sentence (drowning and safety).