Explanation: Local action in a lead-acid battery refers to parasitic electrochemical reactions occurring at the plates due to impurities in the lead or electrolyte. These impurities create small, localized cells that cause the battery to self-discharge even when no external load is connected. * **A) Causes a slow discharge in the cell:** This is the direct result of these unwanted internal reactions, consuming the active material and gradually depleting the battery's stored energy. * **B) Forms a very hard sulfate on the plates:** Prolonged discharge, often exacerbated by local action, can lead to the formation of lead sulfate crystals that become hard and non-conductive, hindering subsequent recharging (a condition known as sulfation). * **C) May be reduced by trickle charging when the battery is not in actual use:** Trickle charging provides a small, continuous current that counteracts the self-discharge caused by local action, maintaining the battery's charge and preventing the formation of damaging hard sulfates. Since all three statements accurately describe effects or remedies related to local action, D is the correct choice.