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

/etc/dpkg/dpkg.cfg is the main configuration file for the dpkg low-level package tool itself, controlling options like verbosity and paths, not repository locations.

/etc/apt/apt.conf stores global APT configuration options such as proxy settings and acquire methods, but not the list of repository sources.

/etc/apt/apt.conf.d is a directory containing snippet files that extend APT configuration, not the file that defines repository sources.

/etc/apt/sources.list contains lines specifying repository types (deb/deb-src), URIs, distribution codenames, and component names that tell APT exactly where to download package index files and packages. Editing this file allows administrators to add, remove, or change the remote mirrors and repositories used during installations and upgrades.

/etc/dpkg/dselect.cfg is the configuration file for the dselect package management front-end and has no role in defining network package sources.