The term “normal” refers to weather data averaged over:

The correct answer is D. Thirty years.

The term “normal” refers to weather data averaged over thirty years. This is because weather patterns can vary significantly from year to year, so it is important to have a long period of data to get an accurate picture of what is “normal” for a particular location.

Option A is incorrect because one day is not a long enough period of time to get an accurate picture of weather patterns. Option B is incorrect because several months is not a long enough period of time to get an accurate picture of weather patterns. Option C is incorrect because one year is not a long enough period of time to get an accurate picture of weather patterns.