Here, Mom, let me help. The actual pressure of the air changes with weather pattern's as I'm sure you know. This has to do with high and low pressure systems from whence all weather related phenomena emanate. However, a given volume of air can be more "dense" than another equal volume of air, and therefore, "weigh" more. The molecules that make up air can be more closely packed together the colder they are. Additionally, the less the volume contains of other materials, such as dust or water vapor, the more closely the molecules will fit together. You can see, from this explanation, the highest "pressure" in this context, will be your example (b)... In my profession of aviation, the aircraft's take-off performance is measurably enhanced on a cold day as opposed to a warm day (shorter take-off distance). The heat has more to do with the density than does water
but both are contributory. Best of luck with the assignment!