a) sets = sets.AbstractEnumerableSet.extend();
b) sets.SingletonSet = sets.AbstractEnumerableSet.extend(…);
c) sets.SingletonSet = sets.extend(…);
d) sets = sets.extend(…);

Answer: b
Explanation: Singleton is an object which can only be instantiated once. The extend keyword is used in class declarations or class expressions to create a class which is a child of another class. The sets object is the namespace for the module, and we define each of the set classes as a property of this object.