What activates a convection current, starting the flow of fluid (a) cold temperatures at the north pole (b) heated liquids at the earth surface (c) iron-rich blocks in the mantle (d) temperatures differences in the mantle

It's the temperature differences in the mantle. Warmer magma near the core rises because it becomes less dense. It reaches the surface, pushes against the tectonic plates, then cools and falls back down to be replaced by new warm magma.