Because of the rain shadow effect, a dry region occurs on the downside of a mountain.
Option: A
Explanation:
In orographic rainfall, mountain acts as barrier and moisture laden wind get an obstacle in its path in the form of that mountain and cause rainfall in the windward sides of the hills (mountain) and create rain shadow region in the lee ward sides of the hills (mountain).
Because when moisture laden winds reach that side all the moisture in the winds dries up. For e.g. eastern part of western ghat mountain in the Peninsular India is an example of rain shadow region.