Unlike light sound cannot travel through a vaccum. An example of a vacum is outer space. You can see lights from space, but can not hear sounds from space
The correct substitutes would be " A. Ellos lo compraron
In English, Ellos compraron El Reloj Pulsera means : They (masculine) bought the watch
Reloj : Masculine singular Pronoun, which means that the objective pronoun should be : LO
Hope this helps
I believe the cerebrum is the largest part of the brain, so the answer should be C.