(b) X = Mg ; Y = MgO ; Type of reaction = Combination
Magnesium ribbon burns in oxygen with a dazzling white flame to form white ash magnesium oxide: $2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$. This is a combination reaction.
The key identifiers are: metal ribbon (Mg is described as a ribbon), dazzling white flame, and white ash (MgO) — all matching the textbook description in Activity 1.1 and Section 3.2.1. The reaction is a combination (two substances combine to form one product), not decomposition or thermal decomposition. Option (d) is wrong because this reaction is exothermic, not endothermic.