The use of constituent demographic data can help explain all of the following options, but it is particularly useful for:
A) determining which groups will or will not vote
Demographic data provides insights into various factors such as age, race, gender, income, and education level, which can influence voting behavior and turnout.
While demographic data can also help in understanding B) election outcomes, C) voter turnout rates, and D) political party membership trends, determining specific groups that are likely to vote or not is a more direct application of that data. Therefore, A) is the best answer in this context.