Explanation

Elemental chlorine exists in nature as two stable isotopes, chlorine-35 and chlorine-37, whose weighted average gives chlorine its atomic mass of about 35.5. Both isotopes have the same atomic number, 17, since the number of protons defines the element; they differ only in neutron count (18 and 20 respectively). Options pairing 35 with 36, or using atomic number 18 (which is argon), are therefore wrong. Hence option D is correct.