Answer:
A. elements of the same kind with different numbers of neutrons
Explanation:
As we know that an atom is represented by
[tex]_z^AX[/tex]
here we know that
z = atomic number
A = atomic number + number of neutrons
now if the number of neutrons in an atom is different but having same number atomic number then the combination of such group of atoms is known as isotopes.
So here we have
[tex]_z^{A_1}X, _z^{A_2}X[/tex]
so above is the example of isotopes