Unix files beginning with which of the following characters are considered hidden?

The exclamation mark has no special hidden-file meaning in Unix; it is used in shell history expansion.

The dollar sign is a shell metacharacter used to reference variable values, not to denote hidden files.

The question mark is a shell glob wildcard matching any single character, not a hidden-file prefix.

The Unix convention treats any file or directory name beginning with a dot (.) as hidden. Tools like 'ls' omit these entries unless the '-a' flag is used, and this behavior is enforced by the shell and filesystem utilities rather than a permission setting.