Most extinction is a. gradual, generally occurring when species cannot adapt genetically to changes in environmental conditions b. the result of slow climate change c. the result of environmental catastrophe d. problematic for generalists e. not often a problem for endemics

An extinction event is a well-known and quick decrease in the biodiversity on Earth. Such an occurrence is recognized by a sharp change in the diversity and abundance of multicellular organisms. It happens when the rate of extinction surges with respect to the rate of speciation. In other words, this happens when species cannot adapt generally to the change in the environment. So the correct answer would be letter a.

phylogenetic tree is the tree of life or evolution tree. it shows the different branches of evolution. the sequence goes like tissues - coelom - backbone - endoskeleton - segmentation