Final answer:
The correct answer is option a. The nasal portion of each eye's retina crosses at the optic chiasm. This crossing ensures that the left and right visual fields are processed by the opposite hemispheres of the brain.
Step-by-step explanation:
In the human visual system, the optic chiasm is where the sorting of optic nerve fibers occurs, allowing visual field information to be processed by the opposite hemisphere of the brain. The fibers that cross in the chiasm are those from the nasal retina of each eye, corresponding to the peripheral visual field.
This means that the axons from the nasal part of the left eye will cross over to the right side of the brain and vice versa for the right eye. Conversely, the axons from the temporal retina do not cross at the optic chiasm but instead project to the same side of the brain, ensuring that the right visual field of both eyes is processed by the left hemisphere and the left visual field is processed by the right hemisphere.