An ionization process involves stripping an electron from an atom - multiple processes involve multiple electrons. The amount of energy required to strip an electron depends on the element and its electronegativity; in essence, atoms that don’t need the extra electron (sodium, for example) give it up for little energy, while atoms that want the electron require a massive amount of energy to take it away (chlorine, for example).
In this case, the energy trend follows so that successive ionizations become more and more energetic regardless of the atom itself, meaning that D is the correct answer for this question.
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