An adjective clause is basically a phrase (to be exact, a dependent clause: a group of words that can't be a sentence) that provides more description. It begins with a pronoun or adverb, like: who, whom, that, which, when, where, or why. It is not necessary to the sentence, so you can take it out and it still makes sense.
Let's work backwards. D is "If I put on headphones, I can play the electric piano without disturbing others." "If I put on headphones" is a phrase, a dependent clause, providing more description. Does it start with one of the words above? No. Is it necessary to the sentence? Yes, because this is the whole point of the sentence: to tell you HOW the character can play the piano without disturbing others, using headphones. D is incorrect.
Here's C: "I have taken piano lessons since I was six years old." "Since I was six years old" provides more description and is dependent. Does it start with one of the words above? No. Is it necessary to the sentence? Yes, because again, it's the point of the sentence. "I have taken piano lessons" doesn't tell you much! C is incorrect.
What about B? "When I first learn a difficult piece, I play it very slowly." "When I first learn a difficult piece" provides more description, and is a dependent clause. Does it start with one of the words above? YES: it starts with "when"!! We're getting closer. But is it necessary to the sentence? Try it: "I play it very slowly." But WHY does the character play it slowly? This phrase IS necessary, to explain, because it's the point of the sentence! B is incorrect.
That just leaves A: "My new electric piano, which includes over 1,000 different settings, is fun to play." The dependent clause providing more description is "which includes over 1,000 different settings." Does it start with one of the words above? YES, it starts with "which"!! Is it necessary to the sentence? Try it without the clause. "My new electric piano is fun to play." That works!! The clause is NOT necessary, starts with "which," AND provides more description! We have our winner!
Answer: A
This passage allows us to see into the inner struggle and loneliness of the character. This is revealed in the phrase "interior gloom" while he faces the "open lattice" but he was not looking at anything at all. He was engrossed in his own thoughts and feelings of a closure or an ending of life implied in the phrase "the fire had smouldered to ashes." The surroundings was so silent and cold as revealed in the words "damp, mild air," "cloudy evening" and "so still."
Having many friends and having few friends is obviously thinking of the amount of friends without caring much about quality. It's good to have few friends, but to have true friends.
It is always preferable to have quality friendships. Friendships built on trust, caring, love, understanding and reciprocity. A friendship between two people is between two people, never a single part. It has to have both parts. It must be a friendship strengthened in personal, intimate, and deep knowledge, to the point of this friendship overcoming difficulties, confusion, misunderstandings, quarrels, everything. This kind of friendship is built with quality. And nowadays few people possess those characteristics or are willing to acquire them. That is why friendship should never be based on quantity, on the largest number of friends. If we have few friends but friends with these characteristics, we know that they are real friends.
Is this a riddle? Because if so, your answer is air. (Hopefully). If not, and you have an actual question, please specify!