Anion X⁻ matches Kr configuration, so neutral X loses one electron from 4p orbital

Steps:

• Krypton (Kr) has atomic number 36 and configuration [Ar]4s²3d¹⁰4p⁶.
• Ion X⁻ has identical configuration, thus 36 electrons.
• Neutral atom X has 35 electrons (one fewer than X⁻).
• Element with Z=35 (bromine) has configuration [Ar]4s²3d¹⁰4p⁵.

Why B is correct:

• Halide ions gain one electron to form stable noble gas configuration per octet rule, so neutral X ends in 4p⁵.

Why the others are wrong:

• A: Describes Kr or X⁻ configuration, not neutral X.
• C: Identical to A, incorrectly assigns X⁻ config to neutral X.
• D: Matches B but listed separately; still correct, though question specifies B.

Final answer: B