The correct option is
C Protoxylem
Primary xylem is formed during primary growth from procambium. It includes protoxylem and metaxylem. The first xylem to develop is called 'protoxylem'. In appearance protoxylem is usually distinguished by narrower vessels formed of smaller cells. Some of these cells have walls which contain thickenings in the form of rings or helices.
So, the correct answer is 'Protoxylem.'.