Need ! what is a defining feature of mammals that is not present in most other vertebrates? a. mammals have a nervous system with a spinal cord. b. mammals reproduce through internal fertilization. c. mammals maintain a constant body temperature. d. mammals can produce milk for their offspring.

Answer is D. Mammals can produce milk for their offspring.

Mammals are the vertebrate which have some distinguishing features.  Female mammals have mammary glands. Mammals got their name from this unique feature. Mammary glands are evolved from the modification of sweat glands. Mammary glands produce milk for their offspring. presence of hair on the body is also a defining feature of mammals. These defining features are not present in most of the vertebrates.    

The answer is D: Mammals reproduce through internal fertilization