I could be wrong on this one, but I think the answer is the first option, The net force on the object is in the direction of motion.
Friction can be acting on the object, but if the other force(s) that are acting on the object are greater, I think the object will still be able to accelerate.
The third option can't be true, according to Newton's second law [tex](f=ma)[/tex]. If there were no forces acting on the object, or if the net forces cancelled each other out, the object wouldn't be accelerating unless its mass was changing.
I suppose the last option is also technically true, but the first is more specific..?
