Final answer:
Paraffin wax is the reagent used to embed tissues during processing, providing a firm support for the tissue that allows for thin sectioning for microscopic analysis.
Step-by-step explanation:
The reagent used to embed tissues during processing is d. Paraffin wax. Tissue embedding is a crucial step in histology where the tissue specimen is surrounded by a medium that solidifies to allow thin sections to be cut. After a tissue sample is fixed in a solution, such as formalin, to preserve its structure, it undergoes dehydration and clearing before being infiltrated with the embedding medium. Paraffin wax is most commonly used because it provides a firm matrix that supports the tissue, allowing for the generation of very thin sections for microscopic examination.