What must be true for two organisms to be considered the same species?

Hello!

Generally, what is considered must be true for two organisms to be considered the same species, is they have to be able to breed and produce offspring between each other.

If a species becomes separated, and over time cannot produce offspring with the other separated group, they are not the same species anymore.

Therefore, your answer is: They must be able to reproduce and produce offspring.

Hope this helped!

 C.  They can produce fertile offspring.