A text file containing a string of information on the hard disk as per…

A cookie is a small text file that a website stores on your computer's hard disk through the web browser. It records browsing-related information such as login state, site preferences and session data, so the site can recognise the user on a later visit. Because it is a stored text file that holds a string of browsing information, the correct answer is Cookie.

The other choices are unrelated: a URL (Uniform Resource Locator) is the web address of a resource, DNS (Domain Name System) translates domain names into IP addresses, and a Plug-in is an add-on software component that extends a browser's features. None of these is a text file that stores browsing information on the hard disk.