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¹²C and ¹⁴C are the isotopes of carbon. An isotope of an element has the same atomic number but the mass number is different.

The number 12 and 14 in the carbon atom refers to the mass number i.e. the total number of neutrons and protons.

Thus the mass number will increase when the number of neutrons increases. So, the answer is (c).

Option (a) is incorrect as increase in the number of electrons does not change the mass number.

Option (b) is also incorrect. If the number of protons increases but not the number of electrons, then the atomic number of the element will change.

Option (d) is not correct as increasing the number of electrons does not change the mass number.