The construction of the sentence in simplest form is--
The Ashoka Pillars.......are a series of columns.
Anything in between are modifiers either for pillars or for Ashoka.
(A) Ashoka, relics of the remorse he had felt after observing the destruction of Kalinga during one of his
quests,- verb "are" is missing after "quests," - INCORRECT
(B) Ashoka are relics of the remorse he felt after observing the destruction of Kalinga during one of his quests,- verb "are" is missing after "quests," - INCORRECT
(C) Ashoka, relics of the remorse he felt after observing the destruction of Kalinga during one of his quests, are - relics of remorse correctly modifies Ashoka and verb "are" is present after quests,-
CORRECT(D) Ashoka are relics of the remorse he felt after observing the destruction of Kalinga during one of his quests, which are- "which" referring to quests is INCORRECT
(E) Ashoka that are relics of the remorse he felt after observing the destruction of Kalinga during one of his quests and are - distorted meaning, "that are relics" - INCORRECT
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