Which sentence contains an adjective clause? a. my new electric piano, which includes over 1,000 different settings, is fun to play. b. when i first learn a difficult piece, i play it very slowly. c. i have taken piano lessons since i was six years old. d. if i put on headphones, i can play the electric piano without disturbing others.

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!

A