In order to move with their gradient, the ions would be moving from where there are more of them to where there are less of them, trying to create a balance in both areas concentration. But the image shows that the potassium ions are in less concentration outside the cell and, still, they are moving into the cell. So it is moving agains its gradient, and A is incorrect.
The sodium ions are moving to the outside of the cell, as shown in the two first proteins that are crossing the membrane. As they are not moving into the cell, B is incorrect too.
Potassium ions are moving into the cell, but not via simple diffusion. Simple diffusion occurs when the ions move into or to the outside of the cell with no "help", only because of the gradient of concentration. In this case, potassium ions are moving against their gradient. Therefore, C is incorrect as well.
There are more sodium ions outside of the cell than inside. Still, sodium ions keep moving from the inside to the outside. Therefore, they are moving against their gradient.
So the correct answer is D. sodium ions to move against their gradient.