The correct option is Magnesium oxide.
Explanation
The burning of magnesium in the presence of oxygen is a classic example of a combination reaction and an oxidation reaction. When metals burn in air, they typically react with oxygen to form metal oxides.
Detailed Analysis:
- Magnesium Oxide (Correct): When a magnesium ribbon is burnt in oxygen (or air), it burns with a dazzling white flame and changes into a white powder. This powder is magnesium oxide ($MgO$). The reaction is represented as:
$2Mg(s) + O_2(g) \rightarrow 2MgO(s)$ - Magnesium Sulphate (Incorrect): This compound ($MgSO_4$) is formed when magnesium reacts with sulfuric acid ($H_2SO_4$), not oxygen alone.
- Magnesium Chloride (Incorrect): This compound ($MgCl_2$) is formed when magnesium reacts with chlorine gas or hydrochloric acid ($HCl$).
- Sand (Incorrect): Sand is primarily composed of silicon dioxide ($SiO_2$), which involves silicon, not magnesium.
Key Takeaway:
Magnesium reacts vigorously with oxygen upon heating to form magnesium oxide, a basic oxide. This reaction is highly exothermic and produces a characteristic bright white light.