Which law states that the volume and absolute temperature of a fixed quality of gas are directly proportioned under constant pressure conditions

That is  Charles' Law. It is true for an ideal gas like helium  or argon.

It is approximately true for other Gases.

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.