fortran-comment-line-start

fortran-comment-line-start is a variable defined in `fortran.el'.
Its value is

"C"

• This variable is safe as a file local variable if its value
  satisfies the predicate `stringp'.

Documentation:
Delimiter inserted to start new full-line comment.
You might want to change this to "*", for instance; or "!" to
allow trailing comments on a line.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 21.1 of Emacs.