Mark's ability to distinguish a trumpet and a trombone or his mother's voice from his sister's voice is most likely due to the of these stimuli.

Mark's ability to distinguish a trumpet and a trombone from each other, or to distinguish his sister's voice form his mother's voice is due to the timbre of these stimuli. Timbre refers to the quality of the sound or tone. Mark is able to distinguish different things from each other, because each object or voice differ from each other's quality, which is why he is able to distinguish the difference.