Bottom left for sure

Either of the right two, it's gotta cover your bits.

I see the tail peeking through so there's a tail hole. Hopefully there is also a poop hatch...although you'd probably have to get a buddy to open it....
- Can a centaur scratch it's butt?
- What would a centaur toilet look like? 🤔
No he can't remove them if he needs to use the bathroom it's clearly option be 3

Does it have man junk and horse junk? It appears to only have horse junk in which case the top right would be correct

Yes, okay, but imagine being a centaur with bottom left pants. Easily accessible pockets are not to be underestimated.
- Pockets, sure, but then you got the ole horse cock flapping in the wind look though
- Surely a centaur would just have saddle bags for easy to access storage.
How do you even put those on? You'd need some kind of helper device
- I was trying to figure that out too. Best I could figure, you'd start by putting the rear legs through both holes then sliding it up to right behind your front legs. Then do some kind of maneuver where you pull your front legs up to your body and try your best to stretch the pants a little bit to get your front legs into the top hole then down and out through the front leg holes, then finish sliding the top hole up into place? Idk I've now thought way too long about how a centaur would actually put pants on.