Introduction:
When a computer is powered on, it goes through a series of startup processes known as booting. Cold booting refers to the process of starting a computer from a completely powered-off state. During a cold boot, the computer's operating system and other necessary files are loaded into memory. In order to perform a cold boot, a specific disk is required.

Explanation:
The disk that is used to cold boot a PC is known as the system disk. This disk contains the necessary files and instructions required to start the computer and load the operating system. Let's understand this in more detail.

System Disk:
The system disk is a storage device that contains the necessary boot files, including the operating system kernel, device drivers, and system utilities. It is typically a hard disk drive (HDD) or a solid-state drive (SSD) that is connected to the computer internally.

During the cold boot process, the computer's BIOS (Basic Input/Output System) performs a power-on self-test (POST) to check the hardware components and ensure they are functioning properly. Once the POST is completed, the BIOS searches for the system disk to load the necessary files to start the operating system.

The system disk is usually partitioned into multiple sections, with one section known as the boot partition. This partition contains the boot loader, which is responsible for loading the operating system into memory. The boot loader reads the necessary files from the system disk and transfers control to the operating system, allowing it to take over the booting process.

Other Disk Options:
- Setup Disk: A setup disk is used to install or configure software or hardware on a computer. It is not specifically designed for cold booting.
- Diagnostic Disk: A diagnostic disk contains tools and utilities that help diagnose and troubleshoot hardware or software issues on a computer. It is not used for cold booting.
- Program Disk: A program disk contains specific software programs or applications that can be installed and run on a computer. It is not used for cold booting.

Conclusion:
In conclusion, the disk used to cold boot a PC is the system disk. This disk contains the necessary files and instructions to start the computer and load the operating system. It is different from setup disks, diagnostic disks, and program disks, which serve other purposes.