Answer
a.
Work Step by Step
The first diagram is one of two different gaseous elements mixed together. We can see that there are two gases here because each gas is represented by a different colored sphere. We know that these atoms are in the gaseous state because they are floating freely, which is what gas particles do.
In the second diagram, one of the two gaseous elements is still in the gaseous state; however, the other gas has become solid, which is represented by the red spheres that are lined up in a rigid arrangement.
This scenario corresponds to option a.
