Concept:Ionization enthalpy is the energy needed to remove an electron from a gaseous atom or ion.Explanation:This process occurs in the gas phase and starts from the ground state of the atom or ion.It represents the energy required to remove one mole of electrons from one mole of gaseous atoms or ions.The general equation is:M(g)→M(g)++e−The amount of energy absorbed in this process is called ionization enthalpy.It is always positive because energy must be supplied to overcome the attraction between the electron and the nucleus.Removing the first electron gives the first ionization enthalpy, while removing subsequent electrons gives higher ionization enthalpies.Answer:B. Ionization enthalpy