Explanation:

A periscope is an optical instrument that uses a system of mirrors or prisms to allow observation of objects not in a direct line of sight. It is commonly used in submarines to see objects on the surface of the water or above the sea level while the submarine remains submerged. Option (A) is correct because periscopes are primarily designed for viewing objects that are elevated or obscured from direct vision, such as those above sea level from a submerged position. Option (B) is incorrect; while used in submarines, its purpose is to see *above* the water, not under the sea. Option (C) is incorrect as periscopes are not designed for astronomical observation. Option (D) is incorrect; while they can track moving objects, their primary function is not specifically for 'rapidly moving objects' but rather for viewing obscured objects.