The fact is that the atomic number is the number of protons and electrons in an atom and mass number is the number of neutrons and electrons, so it can be sometimes more or sometimes less than the atomic number. For example - Hydrogen atom has 1 proton and 1 electron and thus has atomic and mass number as equal.

Option(a) is not correct as it can’t be less than the atomic number because atomic number is the number of protons and mass number is the sum of protons and neutrons.

Option(b) is not appropriate as it can’t be always greater than its atomic number as because sometimes the number of the number of protons is less as compared to the electrons that is in case of anion.

Option (c) is not appropriate as mass number is not always equal to its atomic number.