About Question com.enthuware.ets.scjp.v6.2.19 :

[ETS User]

Nothing is said about the sub-class. The method can be abstract IF AND ONLY IF the sub-class is marked abstract. Is this really a OCPJP question for Oracle's lawyers or Java Programmers?

[anangphatak]

Nothing is said about the sub-class. The method can be abstract IF AND ONLY IF the sub-class is marked abstract. Is this really a OCPJP question for Oracle's lawyers or Java Programmers?

Also note, It must return a Set can be interpreted as

SubClass extends Class_in_question {
public Set getSet(int a) {...} // method is returning a Set (or anything that IS-A set // covariant returns)
}

[admin]

Hello,
The given answer is correct because as you mentioned there is a way the overriding method can be made abstract. Further, it is not necessary for the return type of the method to be Set as explained in the given comment. That is why this option is wrong.

HTH,
Paul.