Ashark would not be a good index fossil because

The shark cannot be considered as a good indicator for fossil study. Because the species is known to have survived for a very long period of time.  

Explanation:

Fossil studies are done by determining the age of the fossil preserved strata or rock. This is done in order to determine the period of time in which a species found as fossil presence.

Sharks have been known to have survived for a very long duration of time. And, therefore, do not corroborate to a specific period of time.  Graptolites and Ammonites are considered as the best index fossils.

the answer is:

c. islam and christianity

explanation:

islam and christianity are widely practiced in africa. islam is practiced due to its spread from the middle east into africa, and christianity is practiced due to the colonization of africa by the british empire.