Question 6
How do two isotopes of an element differ in their chemical behaviour?
The right answer is the one marked above.
They do not differ. Isotopes behave the same way chemically, because they have the same number of electrons.
Chemistry is decided by the electrons — how many are in the last level, how easily they are lost or gained. Isotopes differ only in neutrons, and neutrons take no part in reactions. So the two chlorine isotopes cannot be separated by any chemical means.