#### Answer

CO2, SO2, HBr

#### 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:
SO2:
S is 32 g/mol and O is 16g/mol so 32 + 2*16 = 64 g/mol
HBr:
H is 1g/mol and Br is 79.904 so 1 + 79.904 = 80.904 g/mol
CO2:
C is 12g/mol and O is 16g/mol so 12 + 2*16 = 44 g/mol
We arrange them in order of increasing molar mass:
CO2, SO2, HBr
and since molar mass is proportional to density we have the same order for the denstiy