Final answer:
The red kangaroo and North American beaver form a monophyletic clade derived from the ancestor at node 3 (Gestation), as they are mammals that give birth to live young and have hair. The platypus, while a mammal, lays eggs and thus does not fit in this clade. The correct option is B.
Step-by-step explanation:
To determine which organisms form a monophyletic clade at node 3, we need to consider the provided figures and descriptions about mammals, specifically the platypus and other mammals with distinct characteristics such as fur or hair.
According to Figure 20.10, animals that belong to the clade with hair are lizards, rabbits, and humans, which are part of the clade Amniota. However, lizards do not actually have hair; they are part of this clade because they descend from a common ancestor that had an amniotic egg.
This means that mammals like the duck-billed platypus, red kangaroo, and North American beaver, which all have hair, are part of a different monophyletic clade derived from the common ancestor at node 3.
The fact that the platypus lays eggs and has features such as a beak and webbed feet, as mentioned in Figure 29.40, highlight its unique evolutionary background. Monotremes like the platypus and echidna are egg-laying mammals, a primitive trait among mammals.
The evolution of hair is a characteristic of more derived mammals. Therefore, the evolution of the amniotic egg preceded the evolution of hair.
Considering the provided information, the organisms that make up the monophyletic clade derived from the ancestor at node 3, presuming 'Gestation' represents the evolution of live birth, would likely include mammals that give birth to live young, which does not include the platypus.
Therefore, the correct answer would be red kangaroo and North American beaver, which are both mammals that give birth to live young and possess hair. The duck-billed platypus does not fit this description because it lays eggs. Option B. is the correct one.