Corpus callosum is the large bundle of nerve fibers that serve to connect two cerebral hemispheres. It is present in brains of placental mammals but the amphibians like frogs lack it. Thus, the correct answer is option D.

Introduction:
The brain of a rabbit and a frog differ in several ways. One of the key differences is the presence of the corpus callosum in the rabbit's brain. The corpus callosum is a bundle of nerve fibers that connects the two cerebral hemispheres and allows for communication between them. This structure is absent in the frog's brain.

Explanation:
The brain of a rabbit differs from that of a frog in several aspects. Let's examine each difference in detail:

1. Large Olfactory Lobe:
The olfactory lobe is responsible for processing smell. In rabbits, the olfactory lobe is relatively large compared to frogs. This is because rabbits heavily rely on their sense of smell for survival, as they are prey animals. A well-developed olfactory lobe allows rabbits to detect predators or locate food sources more efficiently.

2. Small Hypothalamus:
The hypothalamus is a vital part of the brain that regulates various physiological functions such as body temperature, hunger, thirst, and hormone production. In rabbits, the hypothalamus is relatively small compared to frogs. This may be due to differences in the physiological needs and behaviors of these two animals.

3. Small Cerebellum:
The cerebellum is responsible for coordinating movement, balance, and posture. In comparison to frogs, rabbits have a relatively small cerebellum. This may be because rabbits are adapted for swift and agile movements, while frogs have a different locomotor strategy due to their anatomical structure.

4. Absence of Corpus Callosum:
The most significant difference between the brains of rabbits and frogs is the presence of the corpus callosum. The corpus callosum is a thick band of nerve fibers that connects the two cerebral hemispheres and facilitates communication between them. It allows for the integration of information and coordination of activities between the two hemispheres. In rabbits, the corpus callosum is present, while it is absent in frogs.

Conclusion:
In summary, the brain of a rabbit differs from that of a frog in several ways. These differences include a larger olfactory lobe, a smaller hypothalamus, a smaller cerebellum, and the presence of a corpus callosum. These variations reflect the different needs and adaptations of these two animals in terms of sensory perception, behavior, and locomotion.