This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
I agree that #2 is also correct, in that the term "IS-A" is commonly used to describe a relationship in which the class in question "is a" subtype of another class (useful in polymorphism). Because B implements A in the above example, B IS-A subtype of A and is usable wherever an instance of A is required (or at least *should* be, in principle - pun intended).