Introduction:
The question asks which operating system does not support running more than one program at a time. In this case, the correct answer is option D, which is DOS (Disk Operating System).

Explanation:
DOS is a single-tasking operating system that was commonly used in personal computers before the advent of multitasking operating systems like Windows, Linux, and macOS. Here's a detailed explanation of why DOS does not support running more than one program at a time:

1. Single-tasking nature:
DOS is a single-tasking operating system, meaning it can only execute one program at a time. When a program is running in DOS, it has exclusive control over the system resources, and no other program can run concurrently. This limitation restricts the user from performing multiple tasks simultaneously.

2. Lack of memory management features:
DOS lacks advanced memory management features that are essential for multitasking. It does not have built-in mechanisms to allocate memory efficiently among multiple programs or to protect one program's memory from being accessed by another program. As a result, running multiple programs simultaneously in DOS would lead to conflicts, crashes, and unpredictable behavior.

3. Cooperative multitasking:
Even though DOS does not support true multitasking, it does provide a limited form of multitasking called cooperative multitasking. In cooperative multitasking, programs voluntarily yield control to other programs to allow them to run. However, this requires explicit cooperation from the programs themselves, and if a program does not yield control, it can monopolize system resources, preventing other programs from running.

4. Lack of process management:
DOS does not have a sophisticated process management system like modern operating systems. It does not have the concept of processes or threads that can be independently scheduled and executed. Instead, it relies on the user to manually start and terminate programs, without any built-in mechanisms for managing their execution.

Conclusion:
In conclusion, DOS does not support running more than one program at a time due to its single-tasking nature, lack of memory management features, limited multitasking capabilities, and absence of process management. Multitasking and the ability to run multiple programs concurrently became possible with the introduction of modern operating systems like Windows, Linux, and macOS.