The layer of epidermis where most cells have stopped dividing and started making large amounts of keratin is the

Stratum Granulosum

Explanation:

The stratum granulosum is a layer of the epidermis and consists of four to six cell layers. The keratinocyte appearance changes in the cell layers of stratum granulosum as some cells stop dividing and the process of keratinization starts. Keratinization includes filling the cells with the protein keratin. During keratinization, the cells flatten, and the disintegration of their nuclei and organelles occur.

Due to the loss of the nucleus, these cells cannot divide and become dead. The keratinized cells accumulate keratohyalin granules and lamellar granules. The cells of this layer have thick plasma membrane due to the binding of cytosol proteins bind to the inner membrane face and coating of the external surface of the membrane by lipids of lamellar granules.