Explanation:

All of the above

Thermal action: Rocks expand and contract in response to changes in temperature, known as thermal action. This expansion and contraction can contribute to the physical weathering of rocks.

Hydration: Hydration involves the absorption of water by minerals within rocks. The process of hydration can lead to the expansion of minerals, causing physical stress and potentially contributing to the breakdown of the rock.

Crystallization: Crystallization refers to the formation of crystals within the rock. As minerals crystallize, they may occupy more space than the original minerals, leading to expansion and contributing to the fracturing or disintegration of the rock.

Therefore, all of the above (thermal action, hydration, and crystallization) can contribute to the expansion of salts in rocks.