Possessive adjectives go before nouns. In 'This is my book,' 'my' comes before the noun 'book'. The other options use possessive pronouns (hers, theirs, ours) which are not followed by nouns.
Class 6 › English Grammar and Composition › Chapter 4: Possessives › Topic: Possessive adjectives: definition and usage (belongings, relationship, body parts, connected things)
Aligned to the NCTB national curriculum.