What is the thin sheet that covers the camera's charge-coupled device?

The thin sheet that covers the camera's charge-coupled device (CCD) is indeed referred to as the shutter. The shutter is a crucial component in photography that controls the exposure time of the sensor to light, allowing a photographer to capture images at varying light conditions. When the shutter opens, light can hit the CCD, and when it closes, it stops the exposure to light.

Although options like lens cap, filter, and viewfinder are associated with camera functions, they serve different purposes. The lens cap is used to protect the lens from damage or dirt when the camera is not in use. A filter is primarily used to modify the light entering the lens or to protect the lens. The viewfinder, on the other hand, helps the photographer compose the shot but does not play a direct role in covering the CCD. Thus, the term "shutter" specifically relates to the mechanism that covers and reveals the CCD to control exposure, making it the correct answer for the question.