Isoelectronic ions share the same electron configuration as noble gas atoms.

Steps:

• Kr has atomic number 36 and configuration [Ar]4s²3d¹⁰4p⁶, with 36 electrons.
• X⁻ has the same configuration, so X⁻ also has 36 electrons.
• Neutral atom X has one fewer electron than X⁻, totaling 35 electrons.
• Element with 35 electrons is Br, with configuration [Ar]4s²3d¹⁰4p⁵.

Why B is correct:

• B matches Br's configuration, where adding one electron forms Br⁻ isoelectronic with Kr (same 36 electrons, per Aufbau principle).

Why the others are wrong:

• A: Matches Kr itself, so X⁺ would be isoelectronic, not X⁻.
• C: Identical to A; incorrect for neutral X.
• D: Matches Se (34 electrons); Se²⁻ is isoelectronic with Kr, not Se⁻ or similar.

Final answer: B