Which of the following best explains ways in which fish and mammals differ

Fish are ectothermic and lay eggs to reproduce, while mammals are endothermic and give live birth to offspring.

Through the elimination process: 
Fish are invertebrates and have scales and paired fins, while mammals are vertebrates and have fur or hair. - Fish are vertebrates! 
Fish are endothermic and have gills, while mammals are ectothermic and have lungs. - Fish are ectothermic (cold-blooded), and mammals are endothermic (warm-blooded)! 
Fish have streamlined bodies and three-chambered hearts, while mammals are bipedal and have four-chambered hearts. - Fish have four chambered hearts and not all mammals are bipedal.

This leaves choice: Fish are ectothermic and lay eggs to reproduce, while mammals are endothermic and give live birth to offspring. All of this is true, but it should be taken into consideration that some fish give live birth and some mammals lay eggs.