Even Carnot engine cannot give 100% efficiency because we cannot :

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prevent radiation

find ideal sources

reach absolute zero temperature

eliminate friction

answer is C.

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Detailed Solution

The efficiency of Carnot engine is ,

$η = 1 - \frac{T_{2}}{T_{1}}$

Where, $T_{1}$ is the temperature of the source and $T_{2}$ that of sink.

Since, $\frac{T_{2}}{T_{1}} = \frac{Q_{2}}{Q_{1}}$

So, $η = 1 - \frac{Q_{2}}{Q_{1}}$

To obtain 100% efficiency (i.e., $η = 1$ ), $Q_{2}$ must be zero that is, if a sink at absolute zero would be available, all the heat taken from the source would have been converted into work.

The temperature of sink means a negative temperature on the absolute scale at which the efficiency of engine is greater than unity. This would be a violation of the 2^nd law of thermodynamics. Hence, a negative temperature on the absolute scale is impossible. Hence, we cannot reach absolute zero temperature.

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