Strongest oxidizing agent has highest electron affinity (tendency to gain electrons).

Steps:

• Identify elements: A (Ne, noble gas, period 2), B (Mg, group 2 metal), C (Ar, noble gas, period 3), D (Ca, group 2 metal).
• Recall oxidizing agents gain electrons; strength depends on electron affinity (EA), where more negative EA indicates stronger oxidizer.
• Metals (B, D) have positive EA (endothermic), so poor oxidizers.
• Noble gases (A, C) have near-zero but negative EA; Ne > Ar due to smaller atomic size and higher effective nuclear charge.

Why A is correct:

• Ne has the most negative EA among options, per periodic trend where EA increases across periods for similar configurations.

Why the others are wrong:

• B: Mg loses electrons easily (reducing agent), low EA.
• C: Ar larger than Ne, less negative EA.
• D: Ca loses electrons easily (reducing agent), low EA.

Final answer: A