Explanation: When a lead-acid battery is being overcharged, the excess electrical energy causes electrolysis of the water in the sulfuric acid electrolyte. This process breaks down water (H2O) into its constituent gases: hydrogen (H2) and oxygen (O2). **D) Emission of hydrogen gas.** Hydrogen gas is highly flammable and extremely explosive, especially when mixed with air in certain concentrations. This poses a significant safety risk, as a spark or flame can cause an explosion, which is harmful to humans. **A) Internal plate sulfation may occur under constant charging.** Sulfation primarily occurs when a battery is discharged and left in that state, allowing lead sulfate crystals to harden. While overcharging can cause other battery damage (like grid corrosion), sulfation is not a typical effect of *constant charging* and is a battery health issue, not a direct human harm like an explosion. **B) Emission of oxygen.** Oxygen gas is also produced during electrolysis. While oxygen supports combustion, it is not flammable or explosive itself. The primary explosive hazard comes from the hydrogen gas. **C) Emission of chlorine gas.** Chlorine gas is not produced in lead-acid batteries. The electrolyte is sulfuric acid, which does not contain chlorine. Chlorine gas is highly toxic and corrosive but is chemically irrelevant to lead-acid battery operation.