The correct option is C Calyptrogen
According to histogen theory, as proposed by Hanstein, the root and shoot apices have three distinct meristematic regions called as histogens. These are namely; dermatogen, periblem and plerome. The dermatogens make uniseriate epidermis by anticlinal divisions, periblem forms cortex and plerome serve as the progenitor of stele (pericycle, vascular bundles, pith rays or medullary rays and pith). Monocotyledons have a fourth histogen, calyptrogen; being the fourth one, that makes root cap.
Thus, the correct answer is option C.