ANS:C - surface evaporation of unbound moisture occurs.

In the constant rate period of the rate of drying curve for batch drying, the correct statement is: Surface evaporation of unbound moisture occurs. During this period, the rate of drying is limited by the rate at which moisture can evaporate from the surface of the solid material. The moisture content of the material is high enough that it saturates the surface, creating a thin film of liquid water. As a result, the drying process is primarily governed by the heat transfer from the surroundings to the surface of the material and the subsequent evaporation of moisture from the surface. Cracks on the surface or a sudden decrease in the rate of drying are not typically observed during this period.