Human Body

Step 1: Analyze the options.

• (1) Stethoscope: A stethoscope is used to listen to sounds within the body (e.g., heartbeats). It does not use light. This option is incorrect.
• (2) Centrifuge: A centrifuge is used to separate components of a liquid based on density. It is unrelated to examining internal organs. This option is incorrect.
• (3) Sterilizer: A sterilizer is used to kill microorganisms on medical instruments. It does not use light to examine internal organs. This option is incorrect.
• (4) Optical fibre: Optical fibres are used in endoscopes, which allow doctors to examine internal organs by transmitting light and images through the fibres. This option is correct.

Final Answer: Doctors use , which corresponds to option .