Answer: Option E
An operating system (OS) is a software that manages a computer's hardware resources and provides common services for computer programs. An OS can be categorized based on its functionality and structure. One way to categorize OS is based on the number of users it can support simultaneously. Based on this, an OS can be classified as a single-user operating system or a multi-user operating system.
A single-user operating system is an OS that supports only one user at a time. In this type of OS, the computer can be used by only one user, and all system resources are dedicated to that user. Here are some of the features of a single-user operating system:

• The user has complete control over the system.
• The user can install and run any application.
• The user can modify system settings.
• There is no need for security measures to protect data from other users.

DOS, which stands for Disk Operating System, is an example of a single-user operating system. It was popular in the 1980s and 1990s and was used on IBM-compatible personal computers. Here are some of the reasons why DOS is a single-user operating system:

• DOS supports only one user at a time. When the computer is turned on, it boots directly to the DOS prompt, which is a command-line interface.
• DOS does not have built-in security features to protect data from other users. All files and directories are accessible to the user who is currently logged in.
• DOS does not have multi-tasking capabilities, which means that only one program can run at a time.

In conclusion, the correct answer to the question is E. DOS.