#### Answer

Cl2 is the most dense

#### Work Step by Step

We have the formula d = PM/RT where d is the density and M is the molar mass. This means that density and molar mass are proportional so that the larger the molar mass, the more dense the gas is. Therefore we find the molar masses of the gases given:
CO2:
Carbon is 12, O is 16 so we have 12 + 16*2 = 44 g/mol
N2O
Nitrogen is 14, O is 16 so we have 2*14 + 16 = 44 g/mol
Cl2
Cl is 35.45 so 2*35.45 = 70.9 g/mol
Since Cl2 has the highest molar mass, it will be the most dense.