What substance is responsible for maintaining the eyeball's round shape?

The vitreous humor is the correct choice for maintaining the eyeball's round shape. This gel-like substance fills the large cavity between the lens and the retina in the eye. It helps to maintain ocular shape by providing structural support and keeping the retina in place against the back of the eye. The vitreous humor does this by exerting pressure due to its gel consistency, which is crucial for the overall spherical shape of the eyeball.

While the aqueous humor also plays a role in intraocular pressure and provides nutrients to the eye’s tissues, it primarily fills the space in the anterior and posterior chambers in the front part of the eye and is not responsible for maintaining the overall round shape of the eye. Corneal fluid and retinal fluid are not standard terms used to describe components that maintain ocular shape, as the cornea is clear and slightly curved but does not contain a specific "corneal fluid," and retinal fluid refers to the fluid associated with the retina, which is not directly involved in maintaining the shape of the eyeball. Therefore, the vitreous humor is the key substance that contributes directly to maintaining the spherical form of the eye.