The correct answer is a. stratum basale.
The stratum basale, also known as the basal layer, is the deepest layer of the epidermis. This layer is primarily responsible for the rapid cell division that produces new keratinocytes. Cells in the stratum basale are continuously dividing and pushing older cells toward the surface of the skin, where they eventually die and form the outermost layer, the stratum corneum. In contrast, the other layers listed, such as the stratum corneum, stratum granulosum, stratum lucidum, and stratum spinosum, do not have the same level of cell division occurring as the cells in these layers are either in the process of maturing, dying, or are already dead. The papillary and reticular layers, options f and g, are not part of the epidermis but rather the dermis, which lies beneath the epidermis. Hence, the stratum basale is key for skin regeneration and health.