The correct answer is - d. colder and denser.
The main differences between the surface currents and deeper currents are the temperature and density, and that actually decides their position in the water, on the surface, or deeper.
The surface currents are warmer, and because they are warmer the molecules are much more loose, so the density is lower, which in return makes it easier, thus it is in the surface.
The deeper currents are colder, and because they are colder the molecules are much closer and more concentrated, so the density is higher, which means that they are heavier, thus fall deeper below the easier currents.