In a convection current, as the temperature of a fluid increases, its decreases.

In a convection current, as the temperature of a fluid increases, it's DENSITY decreases. 

When a fluid is heated, it expands. With expansion, the density of the fluid decreases and the fluid starts to rise up. As the fluid rises up, it cools and so its density decreases and it starts to fall down.
This process repeats as long as the fluid is being heated. That's why we have a convection current. 

Ithink it is letter b, i may be wrong but i think that is the answer