You are given a fairly old piece of dicot stem and a dicot root. Which of the following anatomical structures will you use to distinguish between the two?

Old dicot root is distinguished from the dicot stem by the position of protoxylem. In stem, protoxylem is towards the pith (endarch xylem) and in the root, it is towards the periphery (exarch xylem). Thus, the correct answer is option C.