How do you represent the nucleus using isotopic notation?

The mass number of an element is the total number of protons and neutrons in the nucleus of a particular isotope of that element. 

In isotope notation, the mass number of an isotope (representing the nucleus) is superscripted before the chemical symbol of the element. 

For example, every hydrogen atom has one proton, but with or without neutrons it has three isotopes. 

Hydrogen-1 (also called protium) has one proton and no neutrons, so it has a mass number of 1. Hydrogen-2 (also known as deuterium) has one proton and one neutron and has a mass number of two. Hydrogen-3 (also known as tritium) has one proton and two neutrons and has a mass number of three. 

To express each hydrogen isotope in isotope notation, describe 1H for protium, 2H for deuterium, and 3H for tritium.

Thus, An element is represented as:

${\mathrm{X}}_{\mathrm{A}}^{\mathrm{Z}}$

Where

X is the chemical symbol of the element.

Z is the atomic number.

A is the mass number.

Hence, the correct option is (A).