The question asks students to translate word equations into balanced chemical equations. Each equation must follow the law of conservation of mass - the number of atoms of each element must be equal on both sides. In reaction (a), calcium hydroxide reacts with carbon dioxide in a combination reaction. Reactions (b) and (c) are displacement reactions where a more reactive metal displaces a less reactive metal from its compound. Reaction (d) is a double displacement reaction where ions are exchanged between two compounds, forming barium sulphate as a precipitate.