Final answer:
Thick sequences of sedimentary rocks form due to simultaneously subsiding basins.
Step-by-step explanation:
The correct answer is d. Simultaneously subsiding basins.
James Hall recognized that thick sequences of sedimentary rocks form due to simultaneously subsiding basins. This means that as the basin sinks or subsides, sedimentation continues to occur, resulting in the formation of thick sequences of sedimentary rocks.
For example, during a period of tectonic activity, if a basin is subsiding due to the movement of the Earth's crust, sedimentation can still occur in the subsiding basin. The deposition of sediments over time can lead to the formation of thick sequences of sedimentary rocks.