Which of the following describes the main structural feature of the lens?

The main structural feature of the lens is that it is a transparent intra-ocular tissue. The lens is a crucial component of the eye's optical system, allowing light to pass through it and focus onto the retina. This transparency is vital for clear vision, as it ensures that light can be effectively refracted without distortion or obstruction. The lens also has a unique structure, consisting of tightly packed lens fibers that are rich in proteins called crystallins, which contribute to its clarity.

While other options touch on various aspects of eye anatomy or vision, they do not accurately represent the lens's primary structural characteristic. For instance, an opaque barrier would hinder vision rather than facilitate it; a flexible muscle relates more to the ciliary body, which adjusts the lens's curvature for focus; and dense connective tissue is characteristic of certain other eye structures, but not the lens itself. Thus, the defining quality of the lens being transparent is what makes option B the correct choice.