White nose syndrome (WNS) is a disease found in bats. The disease, first detected in bats during the winter of 2006, is characterized by the appearance of a white fungus on the nose, skin, and wings of some bats, which live in and around caves and mines. It affects the cycle of hibernation and is responsible for the deaths of large numbers of bats known as the Little Brown Bats.
In the last 6 years, the Little Brown Bats have begun to increase in population in the same areas they lived before. Which statement best explains this increased survival rate?
a. The bats needed to reproduce in greater numbers, otherwise they would have died out completely.
b. Most of the Little Brown Bats adapted immunity to the disease in their lifetimes.
c. The original decline in the bat population due to WNS was a natural occurrence and is part of a natural cycle.
d. A few of the bats possessed an immunity to the WNS disease and produced offspring that were immune.***

I think it's D but they all seem correct.