From what my reading has led me to believe, I think the key word is "dry." It does typically happen more in cool climates because those climates are more apt to have heaters going. Heaters play a big role in "drying" up the air around you in a house.
Even though I don't always use an ESD kit throughout the year (slaps my wrist), I always use an ESD kit when the temperature is low enough to necessitate using the heater. I am forturnate, though. I live in a generally humid climate.
I think the exam expects you to answer, "cool and dry" is most likely for ESD.
For the exam's sake, they expect you to use an ESD kit at all times in your normal duty except with a monitor or other high voltage equipment.