This week's book giveaway is in the General Computing forum. We're giving away four copies of Arduino in Action and have Martin Evans, Joshua Noble, and Jordan Hochenbaum on-line! See this thread for details.
You need to work out the order of invocation of the constructors. Write down which constructor is called and when. Then you can see what of the B object exists. The details are in the Java Language Specification. You will have to follow the links.
By the way: if this is an exam question (original or changed), you must tell us where it is from. It also shows the danger of calling methods not marked final or private from a constructor. It also shows the confusion which can occur when you have two fields of the same name. I hope you are posting this as an example of code you would never use in real life.