The correct option is D perishable
The word 'immortal' refers to living forever, never dying.
Option A, undying refers to lasting forever.
Option B, perishable refers to likely to go bad or decay easily.
Option C, ancient refers to something belonging to the very past and no longer in existence.
Option D, eternal refers to something lasting or existing forever.
Thus, according to the meaning, the word 'perishable' is the opposite of the word 'immortal'.