Thin layer drying is limited to

Correct option is C

Thin layer drying involves drying of grains in a shallow layer. The grain layer should be thin enough for uniform exposure to drying air. In practical post-harvest operations, thin layer drying is limited to about 20 cm depth.
Greater depth reduces drying efficiency and uniformity. Hence, 20 cm is considered the correct limit.
Therefore, option (c) is the correct answer.