Final answer:
The process of keratinization begins in the Stratum granulosum layer, where keratinocytes accumulate keratin and keratohyalin, and transition towards death, eventually forming part of the skin's protective barrier.
Step-by-step explanation:
The layer in which the process of keratinization begins, and in which cells are filled with keratohyalin, is C. Stratum granulosum. This layer is where keratinocytes accumulate large amounts of keratin and keratohyalin, giving the layer its granular appearance. As the cells move further from the blood supply, they begin to die, turning into the flat and protective layers that make up the stratum lucidum (in certain areas like the palms of hands and soles of feet) and the stratum corneum, the outermost layer of the epidermis consisting of dead keratinocytes that form a tough barrier.