Option 1: Conversion of current into sound - This principle is not related to how a stethoscope works. A stethoscope does not convert current into sound.

Option 2: Conversion of sound into current - This principle is also not applicable to a stethoscope. A stethoscope does not convert sound into current.

Option 3: Reflection of sound - This is the correct principle. A stethoscope works by transmitting sound waves from the body to the listener`s ears. The sound waves are reflected by internal body structures, such as the heart or lungs, and amplified through the stethoscope`s tubing to the listener`s ears.

Option 4: Reflection of light - This principle is unrelated to a stethoscope. A stethoscope does not involve the reflection of light.

In conclusion, the correct principle on which a stethoscope works is the reflection of sound.