Which of the Following Is a Fitness Trade-Off

Which of the following is a fitness trade-off?

A) In some hornbill species,the male helps seal the female in a tree with her nest until the young are ready to fledge.
B) Moths are the best pollinators for datura flowers,but bees are the best pollinators for orchids.
C) Some lemmings run into the sea when overgrazing threatens the species.
D) Algal genotypes that can grow rapidly in nitrogen-limited environments are easier for predators to digest.