All of the above. Motion sickness is due to the inner ear message to the brain differing from what your eyes are telling your brain (i.e. your inner ear is feeling 1g - similar to straight and level - and your eyes are watching the horizon tumble). The training gives you generic advice (what to eat, don't eat, hydrate, focus on the horizon, actively fly vs be a passenger, etc) as well as spins you to train your brain to ignore your inner ear messages.

Motion sickness is very common - I think the backseat vomit rate in the F-15 when we were giving incentive/orientation rides was better than 50%. As a T-38 IP; if I had to fly a bunch of backseat rides with ham-fisted students in the summer, I'd have to struggle to not feel bad.

Last note: while it is curable, the common line was if it affected your training. If you had to stop training on so many rides, the problem would have to be addressed. If you could make it through the flight and only feel bad while completing the flight, you were allowed to push a little farther.