Yeah, essentially a centaur is just a creature with 2 torsos on top of one another. Yeah, they also change species halfway through, but just because you connect a human torso to a horse torso doesn't mean the horse suddenly pees from its torso. The human digestion system probably connects directly to the horse digestion system. Maybe it's even got extra stomachs to better process plant material. Either way, there's no reason to think it just pees out of its horse chest - it needs that space to hold the enormous horse lungs that let it gallop.
Probably a centaur will be incontined and impotent if existing in real life, because will be hard for a normal human heart pomping enough blood. The answer is the top right but probably not the actually that make sense in evolution where you wish to have a functional intimate area for reproduction need and of course to piss and take big popo.