At first I thought it was the force of friction acting opposite the direction of motion, but I was told the answer's D. I'm not sure how you can experience a change in acceleration without there being a force present.

If friction acts opposite the direction of motion, how can it make you slide?

There is no interaction force accelerating you, but in the non-inertial frame of the bus there is an inertial force that accounts for that acceleration: http://en.wikipedia.org/wiki/Fictitious_force

I assume the test is done at a level before non-inertial frames are introduced. So better stick to the inertial frame of the road: The bus accelerates backwards, while you just continue to move forward, which doesn't require any forces. That's why you slide relative to the bus.

The only force on you is through your feet on the floor of the bus (and your hand if you were smart enough to hold onto a bar). Since the bus is slowing, those are forces are backwards. And, so, your acceleration is backwards. But you already had speed and momentum forwards. Your motion forwards, relative to the bus, is because you are not accelerating (backwards) quite as fast as the bus.

When you and the bus are heading down the road at 30 km/hr, how fast are you moving? 30 km/hr, of course.

When the bus decelerates to 25 km/hr because of the friction between its tires and the road, how fast are you moving? Still 30 km/hr, of course.

How fast are you approaching the front of the bus? It's moving at 25 km/hr, you're moving at 30 km/hr, it's going to feel as if you've been thrown towards the front of the bus at 5 km/hr, but actually you're just continuing happily (at least until you hit the front of the bus) at the same speed you always were.

The only forces acting on you are friction (between your shoes and the floor of the bus) and if you are unlucky, impact forces when you smash into the front of the bus. These forces are acting to slow you from 30 km/hr to 25 and will eventually get your speed down to that of the bus. But until they do, it will seem as if you arevaccelerating towards the front of the bus; in fact the front of the bus is decelerating while you're not.

Just to add to what has been said, it is quite true that one cannot experience acceleration without a force. If the question had asked: what force causes you to accelerate? the answer wold have been a). As has been pointed out, the acceleration is in the opposite direction of motion.

This QA frustrates me. I would like to add inertial force as a right choice.
D) is an answer if we do not admit concept of intertial force that appears in non IFR. Words "What force" is trickey.
B) can be an answer. Inertial force can be interpreted as gravity in accerelation frame.

Come on people, the answer is obviously D. You can come up with advanced arguments for B, but it's obvious that a student is supposed to put the most proper answer on the test, not the most clever.