What layer of the epidermis represents the transition from living to dead epithelial cells?

The answer to this question would be: Stratum granulosum

In stratum granulosum, the cell is producing keratin and became gradually flattened. The keratinocyte will become a layer of keratin that used for protecting the skin. After a while, the dead cells will be replaced by the new keratin from the deeper layer and dead cells will be shed from the skin.