**Explanation:**

The atomic number (Z) of an atom refers to the number of protons in the nucleus. Here, we will discuss why the correct answer is option 'D' and explain the significance of atomic number in an atom.

**Atomic Number (Z):**

The atomic number of an atom is a fundamental property that determines its identity and place in the periodic table. It is denoted by the symbol 'Z'. Each element on the periodic table has a unique atomic number.

**Protons in the Nucleus:**

Protons are subatomic particles that carry a positive charge. They are located in the nucleus of an atom, which is the central core of the atom. The number of protons in the nucleus is equal to the atomic number of the atom.

**Electrons in the Atom:**

Electrons are subatomic particles that carry a negative charge. They orbit around the nucleus in specific energy levels or shells. The number of electrons in a neutral atom is equal to the number of protons, ensuring that the atom has a balanced charge overall.

**Neutrons in the Nucleus:**

Neutrons are subatomic particles that have no charge (they are electrically neutral). They are also located in the nucleus along with protons. The number of neutrons in an atom can vary, resulting in different isotopes of the same element. Isotopes have the same atomic number (same number of protons) but different mass numbers (different number of neutrons).

**Significance of Atomic Number:**

The atomic number is a crucial characteristic of an atom because it determines the element's identity. Elements are organized in increasing order of their atomic numbers on the periodic table. For example, hydrogen has an atomic number of 1, helium has an atomic number of 2, and so on.

The atomic number defines the unique properties and behavior of an element. It determines the number of electrons in the atom, which influences the atom's chemical reactivity and bonding. It also provides information about the element's position in the periodic table, its atomic mass, and its isotopes.

Therefore, the correct answer to the given question is option 'D' – the atomic number (Z) of the nucleus represents the number of protons in it.