I believe it’s a liquid inside a beaker on a hot Bunsen burner (c)
This is because : Everyday Examples of Convection
Boiling water - The heat passes from the burner into the pot, heating the water at the bottom. Then, this hot water rises and cooler water moves down to replace it, causing a circular motion. Radiator - Puts warm air out at the top and draws in cooler air at the bottom.
Not sure if it’s right tho!