The primary factor that resulted in less available farmland and more privatized farmland during the Industrial Revolution is enclosure.
Enclosure involved the consolidation of smaller plots of land into larger farms, which often resulted in the displacement of peasant farmers who previously used common land for grazing and farming. This process was driven by the desire for more efficient agricultural production and increased yields through the use of mechanical tools and more advanced farming techniques. As land was enclosed and privatized, it became less accessible to the general population, contributing to the decline of communal farming practices and leading to greater concentration of land ownership.
Therefore, the correct response is enclosure.