The chattr +i /etc/important.conf command is correct because it sets the immutable attribute on the file, which prevents any modifications to the file, even by the root user, until the attribute is removed. This is useful for protecting critical system files. The +a attribute only allows appending to the file, not preventing modifications. The +s attribute is used to mark a file for secure deletion, and +d is intended to optimize file deletion, none of which prevent modifications.