A transistor has 3 impurity regions of different doping levels. In the order of increasing doping level, the regions are

• A

  $\text { emitter, base, collector }$
• B

  $\text { collector, base, emitter }$
• C

  $\text { base, emitter, collector }$
• Correct answer
  D

  $\text { base, collector, emitter }$

Hence, the correct answer is option: 'D'.