Step-by-step explanation:
A gas at higher temperature is able to gain more heat from the environment/surroundings and has more kinetic energy to diffuse at a faster rate.
Hence a temperature at -20°C is more ideal.
Molar mass of H2S = 34.07g/mol
Molar mass of HCl = 36.45g/mol
Since H2S has a smaller molar mass, the same number of moles of H2S gas will diffuse faster as compared to the same number of moles of HCl gas.
Hence the answer is Hydrogen sulfide at -20°C. (D)