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.

you didnt add any sort of groups or any options, you can edit it and then people can try to answer now its kinda hard.

hope this gives you a heads up why you arent answered. : d

have a great rest of your day!