When wind creates a sand dune, the sheltered side of the dune

The correct answer is - is more steeply sloped.

When there's winds in the deserts they carry a lot of sand particles with them, and once they start to lose speed, the sand particles start to deposit. As they deposit, they start to create a hill of sand, so called sand dune. The sand will start to accumulate, and as the sand dune grows in size, the accumulation of sand will be much bigger on the side from where the wind comes, while the other side will be partially sheltered and get less send. Because one of the sides will continue to grow, and the other one not, the side from where the wind comes will be much less steeper, while the sheltered side will become much steeper.

a and b because with the other three, they simplify to something else.