Certification answer: Answer (b) is correct. Without a default close operation such as JFrame.EXIT_ON_CLOSE, the Display application will never stop running after the frame is closed -- you must hit CTRL-C to terminate the program.
Answer (a) is incorrect. The frame displays properly even if its size is not explicitly set. The call to the pack() statement in its constructor sets the size based on the size of the JLabel component inside the frame.
Answer (c) is incorrect. Calling the frame's constructor inside the constructor method causes a stack overflow error when the program runs.
Answer (d) is incorrect because (b) is correct.