Explanation: Sulfation refers to the formation of lead sulfate crystals on the plates of a lead-acid battery, which impedes its ability to charge and discharge efficiently. A) **It is a normal process:** During the normal discharge cycle of a lead-acid battery, lead sulfate *does* form on the plates as part of the chemical reaction. This soft, amorphous lead sulfate is easily converted back to lead and lead dioxide during the subsequent recharge, making it a normal, reversible part of operation. B) **Overdischarging:** If a battery is discharged too deeply, an excessive amount of lead sulfate is formed. If the battery then sits in this discharged state for an extended period, these soft lead sulfate crystals can harden and grow into larger, irreversible crystals, drastically reducing the battery's capacity and lifespan. C) **Local action with improper charging:** * **Local action** (also called self-discharge) occurs when impurities in the electrolyte or on the plates cause internal chemical reactions that slowly discharge the battery even when it's not in use. This slow discharge leads to the gradual formation of lead sulfate. * **Improper charging**, particularly consistent undercharging (not fully recharging the battery), leaves some lead sulfate unconverted. Over time, this accumulated lead sulfate hardens. Leaving a battery in a discharged or partially charged state accelerates this hardening. Since all these factors contribute to or are manifestations of sulfation, D) All of the above is the correct answer.