What environmental variable might we use to specify the directory where man page files will be located?

The environmental variable used to specify the directory where man page files are located is MANPATH. This variable allows users to define additional directories that contain man pages which are not included in the default search paths. When the manual command is executed, it checks the specified directories in MANPATH to find the appropriate man pages for the commands or topics being queried.

Other options serve different purposes: PATH is utilized to define where executable files are located for systems to find commands, LD_LIBRARY_PATH specifies the directories for dynamic libraries at runtime, and HOME points to the current user's home directory. Thus, MANPATH is the only variable directly related to locating man page files.