SONAR is mostly used by navigators. SONAR stands for Sound Navigation and Ranging, and it is a technique that uses sound waves to navigate and detect objects in the water.

Option 1: Doctors do not typically use SONAR. SONAR is mainly used in underwater environments, such as in the ocean or lakes, to detect and locate objects, not for medical purposes.

Option 2: Engineers may use SONAR in certain applications, such as underwater mapping, surveying, or research projects. However, it is not the primary field where SONAR is mostly used.

Option 3: Astronauts primarily explore and conduct research in space, which does not involve the use of SONAR. SONAR is specifically designed for underwater environments.

Option 4: Navigators, particularly those who work in maritime operations like sailors, fishermen, and ship captains, heavily rely on SONAR to navigate the water safely. It helps them detect underwater obstacles, determine the depth of the water, and locate fish or other objects.

Therefore, the correct answer is option 4, navigators.