(B) Burning of coal
In burning of coal: $\text{C}(s) + \text{O}_2(g) \rightarrow \text{CO}_2(g)$, two reactants combine to form a single product — this is a combination reaction.
Source: Chapter 1, Section 1.2.1
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The textbook explicitly lists "burning of coal" as an example of a combination reaction (equation 1.15). Burning of methane (option C) is listed as an exothermic reaction, not combination. Digestion of food is decomposition/exothermic. Black and white photography involves decomposition of silver salts. Examiners expect you to recall that combination = two or more reactants → one single product.