Which radar control shortens all echoes on the display and reduces clutter caused by rain or snow?

The correct choice is the fast time constant (differentiator), which serves a specific purpose in radar operation by effectively shortening the duration of echoes on the display. This feature is particularly useful in minimizing the effects of atmospheric disturbances, such as rain or snow. By shortening the echoes, the radar can provide a clearer picture of significant targets by reducing the persistent clutter these weather conditions can produce.

This control works by quickly responding to changes in the incoming signal, allowing the radar system to differentiate between the echoes from moving vessels and the clutter generated by precipitation. The result is a more defined radar image that aids in the interpretation of vessel positions and navigational safety.

In contrast, sensitivity time control is focused on adjusting the radar's sensitivity to different ranges, which can help manage clutter, but it doesn't specifically shorten echoes. Brilliance control adjusts the brightness of the display without affecting the time characteristics of the echoes. Receiver gain control amplifies weak signals but does not directly impact the duration of the echoes shown. Each of these other options plays a role in radar operation but does not provide the same specific benefit in reducing echo clutter as the fast time constant does.