The correct answer is $\boxed{\frac{1}{2}}$.
The ratio of $\frac{{Zp.u}}{{Zp.d}}$ is the ratio of the number of protons in the nucleus of an atom to the number of neutrons in the nucleus of an atom. The number of protons in the nucleus of an atom is called the atomic number, and the number of neutrons in the nucleus of an atom is called the neutron number. The atomic number is always equal to the number of electrons in a neutral atom, and the neutron number is always equal to the mass number minus the atomic number. The mass number is the total number of protons and neutrons in the nucleus of an atom.
The ratio of $\frac{{Zp.u}}{{Zp.d}}$ is important because it determines the stability of the atom. Atoms with a ratio of $\frac{{Zp.u}}{{Zp.d}}$ that is too high or too low are unstable and will decay into other atoms. Atoms with a ratio of $\frac{{Zp.u}}{{Zp.d}}$ that is close to 1 are stable and will not decay.
The options A, B, and D are incorrect because they do not represent the ratio of $\frac{{Zp.u}}{{Zp.d}}$. Option A is the ratio of the number of protons in the nucleus of an atom to the number of electrons in a neutral atom. Option B is the ratio of the number of neutrons in the nucleus of an atom to the number of electrons in a neutral atom. Option D is the ratio of the number of protons in the nucleus of an atom to the number of neutrons in the nucleus of an atom plus the number of electrons in a neutral atom.