The error message indicates that the private key file permissions are too open, meaning that other users can read or write to the file. This is a security risk, as the private key should be accessible only by the owner of the file. To fix this error, the security administrator should use the chmod command to change the permissions of the private key file to 0400, which means that only the owner can read the file and no one else can read or write to it.

The chmod command takes a numeric argument that represents the permissions for the owner, group, and others in octal notation. Each digit corresponds to a set of permissions: read (4), write (2), and execute (1). The digits are added together to get the final permissions for each category. For example, 0400 means that the owner has read permission (4) and no other permissions (0), and the group and others have no permissions at all (0).

The other options are incorrect because they either do not change the permissions at all (D), or they give too much or too little permissions to the owner, group, or others (A, C).

Verified References:

https://superuser.com/questions/215504/permissions-on-private-key-in-ssh-folder

https://www.baeldung.com/linux/ssh-key-permissions