Electric conductivity of the atmosphere is strongest in the ionosphere. The movement of charges is caused by the electric potential difference between the ground surface and the ionosphere as a result of different distributions of electrically charged particles.

The question as phrased is slightly misleading, because thunder itself is not directly caused by the potential difference between the Earth and the ionosphere. Thunder is the sound produced by the rapid expansion of air heated by a lightning discharge. Lightning, in turn, is triggered by the build-up of large electrical potential differences due to charge separation within cumulonimbus clouds (and between the cloud and the ground).

So, while the official “correct” answer (A: the different distribution of electrically charged particles between the ground surface and the ionosphere) points toward the broader mechanism that maintains the Earth-atmosphere electric circuit, it doesn’t directly explain what causes thunder.