Answer:
A) at absolute zero.
Step-by-step explanation:
The efficiency of a Carnot engine is given by the formula:
η = 1 - T2/T1 where T2 is the temperature of the heat sink and T1 the temperature of the source reservoir
Both temperatures are in Kelvin
where T is the temperature of the heat sink and
Therefore for 100% efficiency, η would have to be 1 which means that the heat sink temperature which is at the numerator will have to be 0°K which is absolute zero