Emigration can be defined as the act of leaving one's own country to settle permanently in another
o the variety of life in the world or in a particular habitat or ecosystem
O the action of coming to live permanently in a foreign country
the production and discharge of something, especially gas or radiation

Option D, Phloem

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