Final answer:
At STP, 1 mole each of He and Cl2 gases will have equal volumes as defined by Avogadro's Law but will differ in root-mean-square speed and effusion rate due to the difference in their molecular masses, while having the same average kinetic energy due to the same temperature.
Step-by-step explanation:
When comparing 1 mole of He and 1 mole of Cl2 gases at standard temperature and pressure (STP), the quantity that will be equal for both is their volume. According to Avogadro's Law, equal volumes of gases at the same temperature and pressure contain the same number of molecules, hence one mole of any gas occupies 22.4 liters at STP. Thus, option (d) volume is correct. Options (a) root-mean-square speed, and (b) effusion rate will be different for He and Cl2 because these quantities depend on the molecular mass, and He has a much lower molecular mass compared to Cl2. Finally, option (c) average kinetic energy will also be equal for both gases due to the fact that at STP, gases have the same temperature and the average kinetic energy of a gas is proportional to its absolute temperature, which is the same for both gases in this scenario.