Yo pense que amarillo era el color yellow
The answer is A. Tú tocas el piano. Because Tu tocas is the only correct way of saying you are playing the piano in this instance. C is saying "tocan" which is wrong because it's saying you but then using a the group form of playing. The same thing with B, it's also using the group form of playing and it's past tense. And D is using the I form of playing not the you form.
