It is the district of your parents' legal residence. This is generally the district in which your parent(s) vote. In the vast majority of cases, it is also the district in which you live. Ask your parent(s) -- they probably will know, especially in an election year. If not, it's listed on their voter registration card. You can probably also look it up on the Internet by address.
If your parents are divorced or otherwise maintain legal residences in separate states/districts, you may be able to CHOOSE between them (you can't apply to both). You should consult USNA or your BGO for details.