Concept:The inverse image f−1(y) means the set of all real x such that f(x)=y.Explanation:For f−1(24), solve x2−1=24.x2=25x=±5So, f−1(24)={5,−5}.For f−1(−5), solve x2−1=−5.x2=−4No real x satisfies this.So, f−1(−5)=ϕ.Answer:f−1(24)={5,−5} and f−1(−5)=ϕ, so option B is correct.