In the attachment there is a density column where there is colour Question: tell me why is the red at the bottom of the density column if it is the least dense

I guess that the bottom part that is colored red represents the base of the container and not the actual red liquid.

Always, the densest materials/substances will go to the bottom.

One could see different things if, for example, when one of the elements changes state it becomes more or less dense (an example is the water, which has a solid phase, the ice, less dense than the liquid phase, and you can see that solid water usually floats on liquid water)

But in the text there is nothing about possible changes in temperature or changes in phase, then that option can be discarded.

The only option that makes sense is that the red part at the bottom represents the base of the column.

answer: the answer is a

heat the gas and measure the pressure