What special strip is attached to the edges of window units for adjustment to varying wall thicknesses?

The correct answer focuses on the function of a jamb extension, which is specifically designed to accommodate varying wall thicknesses. Jamb extensions are installed around the window frame and help to create a flush surface between the window and the surrounding wall. This is especially important in construction and carpentry, where the width of walls can differ due to different insulation materials, studs, and architectural design.

A properly adjusted jamb extension ensures that the window is not only operable but also sealed correctly against the elements, thereby enhancing energy efficiency and aesthetic appearance. When walls vary in thickness, the jamb extension provides the necessary adjustment, allowing for a clean installation that meets both structural and visual standards.

While other options such as sill extensions, head trims, and framing strips serve important purposes in window installation and finish, they do not specifically address the need for adjustment to accommodate different wall thicknesses like the jamb extension does.