2. The rainshadow effect causes (a) desertifi-
cation; (b) leaching; (c) natural desert for-
mation; (d) increased precipitation.

Natural desert formation is caused by the rainshadow effect.

Explanation:

The rainshadow effect leads to natural dry land or desert formation along the leeward side of mountain ranges which protects the area from the prevailing winds.

When the prevailing winds rise up the mountain ranges, it becomes cooler as it rises and precipitates as rain along the windward side of a mountain range but leaves the leeward side warm and dry with very less rainfall compared to the windward side.

This causes the areas on the windward side to be moist, lush and fertile but dry and barren on the leeward side forming natural deserts.  

The Gobi Desert, the Atacama Desert, the Colorado Front Range, the Great Basin of Nevada and Utah are natural deserts formed due to rainshadow effect.

b. lithosphere, asthenosphere, mesosphere   is the answer most likely