Growth is the answer you're looking for I think.
I think A is most likely but if not D
Answer:
c) Habitat isolation
Explanation:
Habitat isolation is a mechanism that prevents species from mating even if they don't have reproductive barriers. The differences in the habitat types may thus be sufficient to develop reproductive isolation between two (or more) populations
Free entry is going to be your answer
Protozoans, bacteria, fungi... viruses are not organisms