Please help me with this question:( Endosymbiotic theory is most useful for explaining the development of which of these organisms?

A prokaryotes that could perform photosynthesis
B. single-celled eukaryotes that could gain energy by combining food with oxygen
d
C. multicellular plants and animals that could live either on land or in the water
.
D. single-celled eukaryotes that kept their DNA in a membrane-bound nucleus

Which of the following statements is true about the relationship between genetic variation and natural selection? a. genetic variation must be present in a population before natural selection can act on it. b. genetic variation arises in a population as a result of natural selection. c. natural selection can act on a population whether there is genetic variation within the population or not. d. after natural selection acts on a population, the amount of genetic variation in the population always increases.