1.To construct an instance of a sub class, its super class needs need to be constructed first. Since an instance can only be created via a constructor, some constructor of the super class has to be invoked.
Either you explicitly call it or the compiler will add super() (i.e. no args constructor) as the first line of the sub class constructor.
Now, if the super class ( here, A ) does not have a no args constructor, the call super() will fail. Hence, choices B( ) { }, B(int y ) { j = y; } and B(int y ) { i = y; j = y*2; } are not valid and choice B(int y ) { super(y*2 );
j = y; } is valid because it explicitly calls super( int ) which is available in A.

2.Instead of calling super(...) you can also call another constructor of the base class in the first line (as given in choice B(int z ) { this(z, z); } ). Here, this(int, int) is called in the first line which in turn calls super(int). So the super class A is correctly instantiated.