Strontium ion Sr²⁺ has 36 electrons, matching neutral krypton.

Steps:

• Strontium (Sr) has atomic number 38, so neutral atom has 38 electrons.
• Sr²⁺ ion loses 2 electrons, resulting in 36 electrons.
• Electron configuration of Sr²⁺ is [Kr], the noble gas core.
• Krypton (Kr) has atomic number 36, so neutral Kr atom has 36 electrons and the same configuration.

Why B is correct:

• Krypton's atomic number is 36, identical to the electron count in Sr²⁺, per the principle that isoelectronic species share configurations.

Why the others are wrong:

• A. Calcium has 20 electrons, far fewer than 36.
• C. Rubidium has 37 electrons, one more than Sr²⁺.
• D. Selenium has 34 electrons, two fewer than Sr²⁺.

Final answer: B