Welcome to the Ranch Snowman.
Please read our
name policy and adjust your displayed name accordingly.
Either y=x; or y=null; will make the object pointed to by the variable y eligible for g.c. only if y is the sole variable that refers to that object. As long as there are some alias(*), none of the assignments will make the object eligible for g.c.
Note that at line 18 both x3 and x4 are pointing the same object --due to x4=x3;--. Thus neither A, nor B are valid answers. But at line 18 x5 is the unique reference to the second X3 object --created by X3 x3 = new X3();--, thus either C, or D will make that object eligible for g.c.
(*)more variables pointing to the same object.