Concept: Lightning during thunderstorms helps to add nitrogen to the soil in a form plants can use.
Explanation: Thunderstorms produce lightning, which is a form of electrical discharge.
This discharge causes atmospheric nitrogen and oxygen to combine into nitrogen oxides.
These nitrogen oxides dissolve in rainwater and are carried into the soil as nitrates.
Nitrates are essential nutrients that improve soil fertility for plant growth.
Therefore, the beneficial effect of a thunderstorm on plants is the addition of nitrate to the soil.
Answer: D. adds nitrate to the soil.