The current gain of a transistor is defined as the ratio of the collector current to the A. base current B. emitter current C. supply current D. collector current E. None of the above

The correct answer is B. The current gain of a transistor is defined as the ratio of the collector current to the emitter current. It is a measure of how much current the transistor can amplify. A transistor with a current gain of 100 can amplify a 1mA base current into a 100mA collector current.

The other options are incorrect. Option A is the base current, which is the current that flows into the base of the transistor. Option C is the supply current, which is the current that flows from the power supply to the transistor. Option D is the collector current, which is the current that flows out of the collector of the transistor. Option E is none of the above.