Correct answer: (B) < (A) < (C) < (D)
The first ionization enthalpy generally increases with increasing nuclear charge and decreasing atomic radius. In this case, element B has the least electrons in the outer shell, making it easier to remove an electron, while element D has the most, making it harder to remove an electron, thus establishing the order of ionization enthalpy as (B) < (A) < (C) < (D).