Final answer:
Testosterone and estradiol differ by having different functional groups attached to the same carbon skeleton (Answer B), making them unique compounds with distinct roles in vertebrate biology.
Step-by-step explanation:
Testosterone and estradiol are both sex hormones derived from cholesterol and share structural similarities with it. However, they differ in their molecular structure. The correct answer to the question is B. They have different functional groups attached to the same carbon skeleton. While both testosterone and estradiol have the same molecular formula and are classified as steroid hormones, the difference lies in the type and position of functional groups attached to their carbon skeleton, which result in different physiological roles in the body.