The correct option is A Secondary xylem
In roots, the protoxylem lies towards the periphery and metaxylem lies towards the centre. This arrangement of primary xylem is called exarch. Whereas, in stems, the protoxylem lies towards the centre (pith) and the metaxylem lies towards the periphery. This type of primary xylem is called endarch. Hence, based on the presence or absence of secondary xylem, a dicot root can be differentiated from a dicot stem.