Rather than directly answer, I'll just make some observations. I'll just say this much: you seem to understand what's going on.
Interesting problem, in that it's secretly very dependent on considering friction. Constant speed means the net force is 0. If friction weren't included, then the only force with a horizontal component would be the 75 N force of pulling, which would then be a net horizontal force, and so the horizontal component of velocity wouldn't be constant.
So at 75 N, the net force is 0, and so the frictional force must be 75 N in opposition to the motion. So the question becomes, when you increase to 80 N, does the frictional force change?
If yes, and if it changes to exactly balance the new force, then the resulting velocity would become constant again (So answer = A or B).
If no, then there's now a net force on the object, and so it'll experience an ongoing acceleration (So answer = C).
So answering this requires knowing what determines frictional force, and if it will change with this new pulling force.