When a baby can tell the difference between its mother's voice and other voices, it is distinguishing between two: O timbres O instruments O pitches O volumes

I would think it is pitches

Explanation:

I think the reason why its not instruments is obvious, anyone can make any volume, and I dont know what timbres means sorry.

answer; i believe the correct answer is (e) it is too dangerous just for the sake of promoting ratings;