Which substance can be suitable to treat acidic soil for farming?

Correct answer: Slaked lime

The governing concept is acid-base neutralisation. Acidic soil has an excess of acidic substances, so a suitable treatment should be mildly basic rather than another acid. Slaked lime, calcium hydroxide, is basic and can react with excess acidity, helping raise the soil pH toward a more suitable range. Thus option A is correct. Vinegar, lemon juice, and dilute hydrochloric acid are acidic, so adding them would generally increase rather than reduce soil acidity.