Thermal efficiency of a Carnot engine can approach 100%, only when the temperature of the

Answer: Option C

Since Efficiency of a Carnot engine is $$\aleph =1-\frac{T_2}{T_1}$$
So, when the cold reservoir temperature is zero or when the hot reservoir temperature is infinity we can say that the efficiency is $$100\%.$$
But this is purely an impossible situation even under ideal conditions as given by second law of thermodynamics.