Isobars are atoms of different elements that have the same mass number (total protons plus neutrons) but different atomic numbers (number of protons). Different atomic numbers are why they are different elements.

So the first blank is "atomic" (they differ in atomic number) and the second blank is "mass" (they share the same mass number), giving the pair "atomic, mass". A classic example is argon-40 and calcium-40, both with mass number 40 but atomic numbers 18 and 20.

"mass, mass" is wrong because isobars do not differ in mass number; they share it.

"atomic, atomic" is wrong because the shared quantity is the mass number, not the atomic number, and atoms with the same atomic number would be the same element.

"mass, atomic" reverses the correct meaning; that combination instead describes isotopes (same atomic number, different mass number).

Hence, the correct filling is atomic, mass.