Which substance can be added to improve excessively basic soil?

Correct answer: Organic matter

Excessively basic soil may make several nutrients less available to plants. Organic matter such as compost or manure decomposes through microbial activity and can produce mildly acidic substances, helping lower the soil’s excessive basicity over time. Thus option C is the best answer. Slaked lime and sodium hydroxide are bases, so adding them would not correct alkaline soil; adding more lime would generally intensify the problem.