Answer:
The second one
Step-by-step explanation:
The answer is the second branch.
Reasoning (you can skip this if you want):
Fish are just fish. Fish have evolved from fish and separated into mammals. Scientists can tell this because they all have backbones or just similar bones. Anyways, since fish went one way and mammals went the other, over many years they have evolved into many different species. And sometime, this guy down below evolved into cetaceans (over many, many of years). Cetaceans like dolphins or orcas are all mammals, so it had to have evolved from mammals.
Hope it helped! :>