Elements are types of atoms that are characterized by their atomic number. The atomic number which is the same thing as the number of protons in the atom's nucleus. When this number is changed, a new element is formed.
Isotopes of an element are versions of that element that have diferent number of neutrons. Thus, the atomic mass (which is essentially the mass of the nucleus, or the sum of protons and neutrons) of an isotope changes but the atomic number remains the same.
In the picture that you have included, element D has 6 protons and 7 neutrons. Element 7 protons and 7 neutrons. Since the atomic number (number of protons) has changed, the two elements are different elements and not isotopes of the same element.