C. Pain receptor–tissue injury
Neuroreceptors are found all over one's body and epithelial tissues to detect and be aware of the external stimuli
Receptor proteins are molecules inside a cell or on its surface on which ligands bind. A ligand is any substance (e.g. a hormone, drug, functional group) that binds specifically and reversibly to another chemical entity to form a large complex. Receptor proteins found inside a cell are called internal receptors or intracellular receptors or cytoplasmic receptors.
