Ans. c (Merkel discs). (Ref. Ganong Physiology 21st ed., 125)MECHANORECEPTORS# Four types of mechanoreceptors in glabrous skin are-two in the superficial layers (Meissner's corpuscles and Merkel discs) and two in the subcutaneous tissue (Ruffini endings and Pacinian corpuscles).Meissner's corpuscles# are rapidly adapting mechanoreceptors# respond to Low frequency vibrations, stroking and fluttering types of tactile stimuli.Merkel disc receptors# are slowly adapting receptors# respond to pressure and texture.Pacinian corpuscles# are rapidly adapting and# respond to high frequency vibrations.Ruffini endings# are slowly adapting receptors that respond to skin stretch.# These endings link subcutaneous tissue to skin folds overlying joints.The afferent fibres of these receptors are myelinated and include:# large (group A alpha),# medium (group Ab),# small (group Adelta) diameter fibres.# Smallest (group A gama)- Tactile sensitivity is greatest on the glabrous (hairless) skin of the hands, the soles of the feet, and the lips.