This vision type is active when light availability is neither fully adequate nor inadequate. What is it called?

The correct answer is mesopic vision, which refers to the type of vision that occurs under conditions of intermediate light levels. This state of vision is active when the environment is not sufficiently illuminated for photopic vision (which relies on cone cells and is suited for bright light conditions) or too dim for scotopic vision (which depends on rod cells and is best for low light situations).

In mesopic vision, both cone and rod photoreceptors are utilized, allowing the eyes to adapt to varying light scenarios that fall between night and day. This capability is essential in many situations, such as during twilight or when transitioning from brightly lit areas to dimly lit ones. The involvement of both types of photoreceptors allows for a balance, which is critical for perceiving colors and fine details under these lighting conditions.

When considering the other types of vision: dim vision isn't a formal classification in the context of visual physiology, and photopic vision is specifically aligned with bright light conditions. Meanwhile, scotopic vision is effective in low light but does not account for the presence of any intermediate light levels where mesopic vision predominates.