Technically, we can't answer the question with the given information.
Even though the cart's speed is constant, it may be turning an corner,
or driving along a curved path. If it's not driving in a straight, then there
must be some force acting on it.
If the cart IS driving in a straight line, AND its speed is constant, then
there can't be any net force acting on it. In that case, the correct choice
is ' B '. (I'm sure this is right IF the cart is driving in a straight line.)
