Isotopes are species of same elements having different mass number (proton+neutron) number, they have the same atomic number or proton numbers but differ in numbers of neutrons.

Therefore, the entities (iii)¹⁴₆ X, ¹⁴₇ Y and (iv)⁸₄X, ⁸₅Y are not at all isotopes as they have atomic numbers 6 and 7, 4 and 5 respectively.

Other 2 options (i) ¹²₆ X, ¹³₆ y (ii) ³⁵₁₇X, ³⁷₁₇Y are definitely isotopes.