The radar control used to reduce sea return at close ranges is the __________.

The correct answer is sensitivity time control. This feature is specifically designed to improve radar performance by minimizing the impact of unwanted sea clutter at closer ranges. Sensitivity time control works by adjusting the sensitivity of the radar receiver over time, particularly reducing the gain during the period when echoes from nearby objects, such as water surfaces, can overwhelm the radar signals from distant targets.

By using sensitivity time control, radar operators can achieve clearer images of actual objects of interest, like other vessels or navigation markers, without being distracted by reflections from the sea surface. This is crucial for making accurate assessments while navigating in close quarters, where clutter can lead to potential safety risks.

The other options, while related to radar control, serve different functions. Pulse length control often affects the resolution and detection capabilities of radar but does not specifically target sea return reduction. Fast time constant relates to the radar's ability to manage decay of echoes, which might still allow sea return to interfere with target acquisition. Gain control helps to amplify signals but does not time-manage the receiver's sensitivity to dynamically mitigate close-range sea returns.