In the context of Windows operating systems, what does the error message 'is not recognized as an internal or external command, operable program or batch file.' typically indicate?
a) The specified command is misspelled or does not exist in the system's PATH environment variable.
b) The command prompt does not have sufficient privileges to execute the command.
c) The command is not supported by the current version of Windows.
d) The system's PATH environment variable has been corrupted.