Answer: b) You won't be able to access documents that have not been published online.
Explanation: Public search engines are the used by the public for the searching and retrieving the information from the computer system.It is a helpful web-based service that provides the access to individual .Example-Google search engine etc.
Search engines tend to display the information that can be accessed through online mode.The information that is not present on web will not be accessible by any user.
Other option is incorrect because open service is a advantage of public search engine as it is available to everyone. Thus the disadvantage is option(b).