Which of the following is unique for any given element? a the number of its neutrons b its mass number c its atomic number

For any given element, the atomic number of its atoms is unique. (choice-C)

The atomic number is the number of protons in each and every atom of the element. If the number of protons in an atom of that element changes for any reason, then it becomes an atom of a different element.

i think it would be 5