The correct option is
B dead
An antonym is a word that is opposite in meaning to another word.
Option C: 'Extant' means 'still in existence, still surviving'.
'Dead' is something that is not alive, or not existing anymore.
So, 'dead' is the antonym of 'extant' and option C is correct.
Option A: 'Unheeded' means 'heard or noticed but not regarded'.
Option B: 'Forgotten' means 'unremembered, something that has left your mind/memory'.
Option D: 'Extinguished' means 'put an end to, destroy'.
These words do not mean the opposite of the italicized word 'extant'.
Hence options A, B and D are wrong.