The answer is D. Stethoscope.
A microscope is an instrument used to magnify small objects. A telescope is an instrument used to make distant objects appear closer. A periscope is an instrument used to see above or around an obstacle. A stethoscope is an instrument used to listen to the sounds produced by the body, such as the heartbeat.
A stethoscope is different from the other three options because it is not used to see objects. It is used to hear sounds.
