The correct answer is: A. increase the alkalinity of the soil.
Lime is a liming material that is used to neutralize acidity in soil. It is a common practice in agriculture to apply lime to soil that is too acidic, as this can cause problems for plant growth. Lime can also be used to improve the drainage and fertility of soil.
Option B is incorrect because lime does not increase the acidity of soil. In fact, it does the opposite.
Option C is incorrect because lime does not restore nitrates to soil. Nitrates are a type of nitrogen fertilizer that can be added to soil to improve plant growth. However, lime does not contain nitrates and does not have the ability to restore them to soil.
Option D is incorrect because lime does not make soil more porous. Porosity is a measure of the amount of air space in soil. Lime does not affect the porosity of soil.
In conclusion, the correct answer is: A. increase the alkalinity of the soil.