The correct option is C Protostele
The earliest vascular plants had stems with a central core of vascular tissue. This consisted of a cylindrical strand of xylem, surrounded by a region of phloem. Around the vascular tissue, there might have been an endodermis that regulated the flow of water into and out of the vascular system. Such, an arrangement is termed as a protostele which is the most primitive and simplest type of stele.
Thus, the correct answer is option C.