If the soil of a field becomes too basic, which substance can be useful to add?

Correct answer: Organic manure

Excessively basic soil can be improved by adding organic manure. As organic matter decomposes, it produces humus and mild acidic substances that can gradually reduce the soil's alkalinity. Thus option A is the most appropriate choice in the school-level context. Lime, sodium hydroxide, and washing soda are basic or alkaline materials, so adding them would generally increase rather than correct the basic nature of the soil.