The correct answer is: C. Differential fertility of the land.
Ricardo’s rent principle states that rent arises due to the differential fertility of the land. This means that land that is more fertile will produce more crops, and therefore will command a higher rent. Land that is less fertile will produce fewer crops, and therefore will command a lower rent.
The other options are incorrect because they do not explain why rent arises. Soil fertility is a factor that can affect the productivity of land, but it is not the only factor. Land subsidence is the sinking of land due to natural or human-caused factors, and it can also affect the productivity of land. However, it is not the main reason why rent arises. Supply stability of land is the degree to which the supply of land is constant over time. This can affect the price of land, but it does not explain why rent arises.
In conclusion, Ricardo’s rent principle states that rent arises due to the differential fertility of the land. This means that land that is more fertile will produce more crops, and therefore will command a higher rent. Land that is less fertile will produce fewer crops, and therefore will command a lower rent.