The correct answer is option 'C': Optocoel.

Frogs and mammals belong to different classes of animals, and therefore, there are significant differences in their brain structures. The presence of the optocoel is one such difference between the frog and mammal brains.

The brain is a vital organ responsible for controlling various functions in an organism. It consists of several regions, each with specific roles and functions. Let's explore the key differences between the frog and mammal brain structures.

1. Cerebellum:
- The cerebellum is a distinct region located at the back of the brain. It plays a crucial role in motor coordination, balance, and muscle tone regulation.
- Both frogs and mammals possess a cerebellum, so it is not the correct answer in this case.

2. Corpus Callosum:
- The corpus callosum is a bundle of nerve fibers that connects the two cerebral hemispheres of the brain. It facilitates communication between the left and right sides of the brain.
- In mammals, the corpus callosum is well-developed, allowing for complex integration of information between the hemispheres. However, frogs lack a corpus callosum or any structure performing a similar function.
- Despite the absence of a corpus callosum, frogs can still perform various behaviors and motor functions, albeit with less integration between the two hemispheres.

3. Optocoel:
- The optocoel, also known as the optic lobes, is a distinctive feature present in the frog brain but absent in mammal brains.
- Optocoel refers to a pair of large, rounded structures located at the anterior region of the brain.
- These structures are responsible for processing visual stimuli received from the eyes. They receive visual information and relay it to other parts of the brain for further processing and interpretation.
- In frogs, which are highly visual animals, the optocoel plays a crucial role in coordinating their visual responses and behaviors.

4. Arbor Vitae:
- The arbor vitae is a structure found in the cerebellum of mammals, particularly in the hindbrain known as the cerebellar cortex.
- It is a branching, tree-like pattern of white matter that helps to regulate motor functions and coordinates body movements.
- Frogs do not possess an arbor vitae, as their cerebellum has a simpler structure compared to mammals.

In summary, the presence of the optocoel is the key difference between the brain of a frog and that of a mammal. While both frogs and mammals have a cerebellum, only mammals possess a corpus callosum, and only frogs possess an optocoel.