The first ionization potential of Na is 5.1 eV. The value of electron gain enthalpy of Na+ will be:

The ionization energy of an atom or molecule describes the minimum amount of energy required to remove an electron from the atom or molecule in the gaseous state whereas electron gain enthalpy is the amount of energy released when an isolated gaseous atom accepts an electron to become a monovalent gaseous anion. The first ionization potential of Na is 5.1 eV. This means that 5.1 eV of energy is required to remove one electron from a neutral Na atom to form Na+. The electron gain enthalpy of Na+ is the energy change associated with adding an electron to Na+ to form Na. Since Na+ is already formed by removing an electron, adding an electron back would release the same amount of energy (5.1 eV). Therefore, the electron gain enthalpy of Na+ will be -5.1 eV. However, this value isn't among the given options. There might be a discrepancy in the question or the provided options.