Answer: C (Z's atomic number could be 20). Z forms a positive ion when it reacts with oxygen, so Z loses electrons to form a cation — that means Z is a metal.
Atomic number 20 is calcium, a Group II metal, which forms Ca²⁺: \(2\text{Ca} + \text{O}_2 \to 2\text{CaO}\). This fits a metal that forms a positive ion, so C is correct.
Option A is wrong because Group VII elements are non-metals (the halogens) that gain electrons to form negative ions, not positive ones. Option B is wrong because the most non-metallic element in a period gains electrons to form an anion, the opposite of what Z does. Option D, electronic structure 2,8,6, has 16 electrons and is sulfur, a non-metal that forms the S²⁻ anion. Only a metal such as calcium forms a positive ion with oxygen, so the answer is C.
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