If you were to go north from the Sahel, you'd find . If you'd go south from the Sahel, you'd find . Sahara; Nile
tropical grasslands; Sahara
Sahara; tropical grasslands
Nile; Sahara

This Should Help a Little:

A largely semi-arid belt of barren, sandy and rock-strewn land, the Sahel marks the physical and cultural transition between the continent's more fertile tropical regions to the south and its desert in the north. ... The Sahel also suffers from ethno-religious tensions, political instability, poverty and natural disasters.

the answer is the last option or letter d)