Two atoms are said to be isobars if.

Isobars are the atoms which have an equal mass number (A) but differ in atomic number (Z) i.e. the number of protons, hence (iv) is correct. For example: ¹⁴C₆ and ¹⁴N₇ where both has the same mass number 14 and different atomic number 6 for Carbon and 7 form nitrogen.

Among other 3 options (i) they have the same atomic number but different mass number indicates isotopes.

(ii) they have the same number of electrons but different number of neutrons also talks about isotopes.

(iii)they have the same number of neutrons but different number of electrons is talking about isotones.