Two atoms are said to be isobars if
(a) They have same atomic number but different mass number.
(b) They have same number of electrons but different number of neutrons.
(c) They are same number of neutrons but different number of electrons.
(d) Sum of the number of protons and neutrons is same but the number of protons is different.
The answer is the option (d)
Sum of the number of protons and neutrons is same, but the number of protons is different because isobars have a different atomic number.