The correct answer is - A. Differences in temperature.
The convection currents are the result of differences in temperature. In practice this can easily be seen with the ocean currents. The warm ocean currents from the Equator move towards the Poles and bring to those areas warmer water which is moving on the surface of the ocean because it is lighter. The cold water from the Poles is driven towards the lower latitudes where it sinks deeper because it is heavier. As the cold water reaches the lower latitudes, it warms up, and continues as warm current, and the process circles continuously.