Wind carrying sand grains deposits the sand when the wind
a. speeds up.
b. crosses a depression in the ground.
c. slows down or hits an obstacle.
d. cools after nightfall.

The correct answer is - c. slows down or hits an obstacle.

The sand grains are mostly picked up and distributed by medium to strong winds. Once they start losing on speed on come to some high mountain range they slow down and there is not enough force in them to carry the sand grains anymore so they fall. The force of the wind is a determining factor as to how big of a distance the sand grains will be transported.

Example for slowing down and depositing: the sand grains from Sahara are travelling with strong winds across the Atlantic Ocean and they reach the Amazon rainforest, when they reach it, the thick cover of high trees slows down the wind and the sand grains are deposited.

Example for obstacle and depositing: sand grains from Sahara are transported through Egypt and the Mediterranean Sea, they cross the low flat region of Macedonia in Greece and than hit the Belasica and Kozhuf mountains at the border with the Republic of Macedonia, they slow down significantly and deposit the sand grains near the town of Gevgelija.