For transistor action
(1) Base, emitter and collector regions should have similar size and doping concentrations.
(2) The base region must be very thin and lightly doped.
(3) The emitter-base junction is forward biased and base collector junction is reverse based.
(4) Both the emitter-base junction as well as the base collector junction are forward biased.
Answer & explanation
Correct answer: option 4
For transistor action, the base region must be very thin and lightly doped. Also, the emitter-base junction is forward biased and base-collector junction is reverse biased.