Concept:
Possessive pronouns show ownership and do not use apostrophes.
Step 1: Identify requirement.
\[
{“This book is ___”} \Rightarrow {needs possessive pronoun}
\]
Step 2: Evaluate options.
(A) her's $\times$ incorrect (apostrophe misuse)
(B) hers \checkmark correct possessive pronoun
(C) hers' $\times$ incorrect form
(D) her $\times$ possessive adjective (needs noun after it)