Looking for a synonym of "surya" (sun), Deepika thinks "chaand" (moon) is also a correct answer…

Why

Deepika is grouping words by a shared FEATURE — both shine in the sky at some point — but पर्यायवाची (synonym) requires the words to name the SAME THING, not merely share a resemblance or category. 'सूर्य' and 'चाँद' are two entirely different objects — one a star, one a moon — so being similar (both bright, both in the sky) doesn't make them synonyms, just as 'car' and 'bicycle' aren't synonyms simply because both are vehicles. The genuine synonyms of 'सूर्य' all name the very same object under a different word — 'रवि', 'सूरज', 'भानु', 'दिनकर', 'आदित्य'. Test it: 'रवि उदय हुआ' and 'सूर्य उदय हुआ' describe the identical event; 'चाँद उदय हुआ' describes something else entirely happening. Her brother is right: 'चाँद' is a different object, not a synonym — a पर्यायवाची must name the exact same thing, not just something similar.