The answer to this question is letter B. All three flasks are measured 1 liter and are placed in the same room making them have the same temperature too. All 3 flasks have also the same number of molecules as they contain the same element (Nitrogen) and have the equal volume as well.
This is a need for solving their mass by using the Avogadro’s Law. Following the Avogadro’s Law, Flask A that has the NH3 gases is 17 g/mol, Flask B with NO2 is 46 g/mol and Flask 3 or with the N2 gas is equal to 28 g/mol. Based on the following answers, Nitrogen Dioxide has the largest number of molecules.