Let's analyze the options given:
(I) and (II) are labeled as isobars.

Isobars are atoms of different elements that have the same atomic mass number (same total number of protons and neutrons), but different atomic numbers (different number of protons).

Therefore, (I) and (II) can only be isobars if they are atoms of different elements with the same atomic mass number.
(III) and (IV) are labeled as isotopes.

Isotopes are atoms of the same element that have the same atomic number (same number of protons) but different atomic mass numbers (different number of neutrons).

Therefore, (III) and (IV) are isotopes if they represent atoms of the same element with different numbers of neutrons.

Let's determine which statements are correct:

I & II are isobars: This is correct if (I) and (II) are atoms of different elements with the same atomic mass number.

I & II are isotopes: This is incorrect because isotope relationships cannot exist between atoms of different elements.

III & IV are isotopes: This is correct if (III) and (IV) are atoms of the same element with different numbers of neutrons.

Both a and c: This is the correct answer if (I) & (II) are isobars and (III) & (IV) are isotopes.

Therefore, after analyzing the options and the definitions of isobars and isotopes, the correct answer is:
Both a and c