Answer
The correct option is (C) number of neutrons
Work Step by Step
Isotopes are atoms of same elements that differ in atomic masses. Different isotopes of a single element occupy the same position in the periodic table. They have the same number of protons, but they vary in their neutrons. The mass number of an atom is the sum of its protons and neutrons. For example, carbon exists in three isotopes 12C6, 13C6,14C6 which contains 6 neutrons for carbon 12, 7 neutrons for carbon 13, and 8 neutrons for carbon 14. Therefore, the isotopes differ in the number of neutrons.
Isotopes are the elements which have the same number of electrons and protons, but a different number of neutrons. Therefore, the options a and d are incorrect.
The atomic number is nothing, but the number of electrons and neutrons and in the atom. Therefore, the option (b) is also an incorrect option.
Hence, the correct option is (C) number of neutrons.
