The correct answer is - afternoon thunder showers.
When we say stable atmospheric conditions it means that the atmosphere should be relatively calm, without having any disturbances in it. A nice example for a stable atmospheric conditions is a clear sunny sky and a calm wind. On the other hand, we have the afternoon thunder showers. The afternoon thunder showers are the total opposite of stable atmospheric conditions. When they occur, there's lot of disturbances, air pressures are mixing, the winds are changing speeds, heavy rainfall occurs, the air pressure changes rapidly, there's electrical charges, all in all creating a real mess in the atmosphere.