Earthquakes are primarily caused by the movement of tectonic plates, specifically at plate boundaries where these plates interact. The most accurate explanation among your options is:
A. Blocks of crust slide against one another, often at plate boundaries.
When these blocks of crust, or tectonic plates, grind against each other, stress builds up due to friction. Eventually, this stress can exceed the strength of the rocks, leading to a sudden release of energy in the form of seismic waves, which we feel as an earthquake.
While pressure can build up in molten rock beneath the Earth's crust, it is not the main cause of most earthquakes. Some earthquakes, known as volcanic earthquakes, can occur due to volcanic activity, but the sliding of tectonic plates at boundaries is the most common cause.
So, the best description of the cause of earthquakes in your options would be A.