Given: Which constructor initializes the variable x3?

The default constructor of class X (a superclass) is responsible for initializing variables declared in X, not variables declared in its subclasses like Z.

The no-argument constructor of class Y (an intermediate superclass) is responsible for initializing variables declared in Y, not variables declared in its subclass Z.

The constructor of class Z is responsible for initializing variables declared within class Z, including `x3`. Whether `x3` is initialized with a default value, an explicit initializer at declaration, or an assignment within the constructor body, this process occurs during the execution of Z's constructor.

The default constructor of the `Object` class (the root class) initializes fundamental object properties common to all objects but has no knowledge of specific instance variables like `x3` defined in a subclass.