Five elements A, B, C, D and E have the following atomic numbers. Among these elements, which element belongs to third period and has the highest ionisation potential?

The minimum amount  of energy is required to pull out an electron from an atom or molecule in the gaseous state.
The first ionisation energy is the energy required to remove the most loosely held electron from one mole of gaseous atoms to produce 1 mole of gaseous ions, each with a charge of 1^+.

X_(g)------------>X⁺+e^-
It is the energy needed to carry out this change per mole of X.