Which corneal layer facilitates transparency through its pump action?

The endothelium is the correct answer because it plays a crucial role in maintaining corneal transparency through its regulatory functions. The endothelial cells are responsible for pumping excess water out of the stroma, which is the thickest layer of the cornea. This pump action is vital because any excess fluid in the stroma could lead to swelling, causing loss of transparency and blurred vision.

The endothelium operates by utilizing a pump mechanism that removes fluid from the stroma and maintains the necessary balance of water content, allowing light to pass through the cornea effectively. This is essential for the cornea's transparency, which is necessary for optimal vision.

The other layers, while important to the overall structure of the cornea and its function, do not have the same role in facilitating transparency through a pump action. The stroma provides structural integrity and strength but is not directly responsible for transparency. Bowman's layer acts as a protective barrier, and Descemet's membrane serves as a basement membrane for the endothelium but does not involve itself in fluid regulation or the transparency of the cornea.