Even Carnot engine cannot give 100% efficiency because we cannot

  1. eliminate friction
  2. reach absolute zero temperature
  3. prevent radiation
  4. find ideal sources


Efficiency of a carnot engine is given by

η = 1 - Q2/Q1

In order to achieve 100% efficiency (η=1), Q2 must be equal to 0 which means that all the heat form the source is converted to work.

The temperature of sink means a negative temperature on the absolute scale at which the temperature is greater than unity. This would be violation of second law of thermodynamics. Hence negative temperature of absolute scale is impossible and we cannot reach absolute 0 temperature.

The correct option is B.