Answer:
Option D, Phloem
Step-by-step explanation:
Phloem consists of following parts -
a) Sieve tubes
b) Companion cells
c) Phloem fibers
d) Phloem parenchyma cells
Protophloem and metaphloem matures before and after cell elongation respectively. However, As a plant ages , the sieve tubes of protophloem get destroyed as a plant ages while the remaining cells in the phloem converts into fibers. These fibres and later maturing metaphloem appear as tissue with in the bark of the tree.
Hence, option D is correct