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The stratum granulosum is a crucial layer of the epidermis primarily composed of a granular layer of cells. This layer is characterized by the presence of keratohyalin granules, which play a significant role in the process of keratinization. As cells move up from the deeper layers of the epidermis, they begin to flatten and accumulate these granules, which contain proteins that contribute to the formation of keratin, essential for the protective properties of the skin.

The cells in the stratum granulosum are typically alive and are transitioning into a more keratinized form as they prepare to become part of the outer protective barrier of the skin. This distinguishing trait of granular cells is crucial in understanding the role of different skin layers in maintaining skin health and function. The presence of these granules is a defining characteristic of this skin layer, making it the correct answer in identifying what the stratum granulosum is primarily composed of.