Answer: (E)
Only (E) directly casts doubt on Chopin’s playing ability, “more difficult pieces…had he the ability to play them.”
Even if Chopin never played other composer’s works, it isn’t necessarily because those works were too difficult. We need something stronger than (A).
The issue is Chopin’s piano playing ability. So that he could or could not play other instruments is unimportant. (B) is out.
(C) does not relate to issue. Even if his etudes were Chopin’s most difficult pieces that doesn't diminish his piano playing prowess.