Which file should be edited to select the network locations from which Debian installation package files are loaded?

`/etc/dpkg/dpkg.cfg` is a configuration file for dpkg itself, defining options for how dpkg operates, not for package sources.

`/etc/apt/apt.conf` is the main configuration file for APT, but it typically holds general APT settings rather than the list of package sources.

`/etc/apt/apt.conf.d` is a directory containing supplementary configuration files for APT, which are loaded in alphabetical order, but the primary package source list is in `sources.list`.

The `/etc/apt/sources.list` file lists the APT repositories from which packages are downloaded and installed. Each line in this file typically specifies a repository URL, the distribution name, and components (e.g., main, restricted).

`/etc/dpkg/dselect.cfg` is a configuration file specifically for the `dselect` package management frontend, not for defining APT package sources.