The line A is valid in a configuration file in /etc/pam.d/ because it follows the correct syntax and semantics of a PAM configuration file. The syntax of a PAM configuration file is:
module_interface control_flag module_name module_arguments
The module_interface specifies the type of authentication action that the module can perform. There are four types of module interface: auth, account, password, and session. The control_flag specifies how important the success or failure of the module is to the overall authentication process. There are four types of control flag: required, requisite, sufficient, and optional. The module_name specifies the name of the library that contains the module. The module_arguments are optional parameters that can modify the behavior of the module.
The line A has the following components:
module_interface: auth, which means the module is used for user authentication.
control_flag: required, which means the module must succeed for authentication to continue. If the module fails, the failure is recorded and the rest of the modules are executed.
module_name: pam_unix.so, which means the module is the standard Unix authentication module that verifies the user’s password against the /etc/passwd and /etc/shadow files.
module_arguments: try_first_pass nullok, which means the module tries to use the password that was previously entered by the user (if any), and allows users with empty passwords to log in.
The line B is invalid because it has the wrong order of the fields. The control_flag should come before the module_name, not after. The line C is invalid because it uses parentheses and colons instead of spaces to separate the fields. The line D is invalid because it has the wrong syntax for the control_flag. The control_flag should be a single word, not a word in parentheses.
