By
A convection current is caused by differences in
a) temperature
b) mass.
c) air pressure.
d) color.

A convection current is caused by differences in  "temperature".

Option: A

Explanation:

The convection current is the outcome of variations in temperature. Like hot substance which is less denser, it rises while cool material which is more denser, it sinks. It is this motion that produces circulating processes in the environment, in water and in the Earth's mantle identified as convection currents. When air heats up it increases in the atmosphere, enabling colder air to circulate in below. This air flow generates waves, together with the Earth spinning. In effect, winds produce surface waves over the ocean.