(B) Al₂O₃ and MgO
When aluminium burns in air: $4\text{Al} + 3\text{O}_2 \rightarrow 2\text{Al}_2\text{O}_3$; when magnesium burns in air: $2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$.
Source: Chapter 3, Section 3.2.1
The correct formulae come from the valencies: Al has valency 3 and O has valency 2, giving Al₂O₃; Mg has valency 2 and O has valency 2, giving MgO. Options with Al₃O₄ or MgO₂ are chemically incorrect. The textbook explicitly states the reaction $4\text{Al} + 3\text{O}_2 \rightarrow 2\text{Al}_�\text{O}_3$ and magnesium oxide (MgO) is standard knowledge from the activity series chapter.