Iron nails were dipped in an aqueous solution of copper sulphate. After about 30 minutes, it was observed that the colour of the solution changed from

This is a single displacement reaction where iron ($Fe$), being more reactive, displaces copper ($Cu$) from copper sulphate solution ($CuSO_4$). The reaction is: $Fe(s) + CuSO_4(aq) \rightarrow FeSO_4(aq) + Cu(s)$. The initial copper sulphate solution is blue. As the reaction proceeds, ferrous sulphate ($FeSO_4$) is formed, which is light green in color. The provided NCERT text in Activity 1.5 confirms that ferrous sulphate crystals are green, and the text also notes that a 'change in colour' is a key observation indicating a chemical reaction has occurred. Distractor A is wrong because the solution is not initially colourless; C is wrong because it doesn't become colourless; D is the reverse of the actual observation.