laura sees a horse pulling a buggy. she wonders how it can accelerate if the action of the horse pulling the cart would causes an equal and opposite reaction of the cart pulling on the horse. which explanation best answers her question? the net forces exerted on the the horse and cart are the same, but the action force is larger. the net forces exerted on the the horse and cart are the same, but they are in the same direction. the net forces exerted on the the horse and cart are not the same, so the two forces cancel out. the net forces exerted on the the horse and cart are not the same, so they are not balanced forces.

I'm not happy with any of the choices.
The last one is trying hard to be the right answer, but it's
worded so awkwardly that it can easily be misunderstood.  

The way I like to answer Laura's question is like this:

-- Yes, the action is the horse pulling on the cart.
-- Yes, the reaction is the cart pulling on the horse.
-- Yes, the action and reaction have equal strength, and they're in opposite directions.
-- But they act on TWO DIFFERENT OBJECTS ... the horse and the cart.

Yes, two equal and opposite forces cancel each other and
add up to zero if they act on the SAME OBJECT.
Nobody ever said that they cancel each other and add up
to zero if they act on TWO DIFFERENT OBJECTS.   
There's nothing here to be worried about.