Question: 2.The question asks about the primary purpose of the passage, which implies we have to choose an option which hints to most parts of the passage.
A. Darwin's theory of evolution cannot explain the rapid emergence and proliferation of flowering plants.
This is easily eliminated as the author just mentions Darwin, which is not to criticise.
B. compare the prevalence of angiosperms to that of ferns and conifers.
Again, it was just a part, and this comparison existed to establish the efficiency of angiosperms (as they prevailed very quickly and captured 90% eventually).
C. discusses the reasons a wide variety of species evolved within the angiosperm group.
Most people chose this. We do know the reasons some species evolved efficiently, but there is no reason WHY they evolved under the Angiosperm group. This does not exist.
D. Explain how flowering plants evolved smaller genomes. present evidence suggesting an explanation for the
Evolution is a mystery. Yes, they had it, and that is established via research, but we do not know how they evolved smaller genomes.
E. remarkable efficiency and productivity of flowering plants
CORRECT. This passage exists to tell us in many ways that they are basically the best. The way he mentions counterarguments makes it more clear, and if you read it again, you will feel the same.
Note - This is just my reasoning! which got me correct here.