Introduction:
When a CPU is operating in user mode, it has restricted access to certain resources and instructions. Switching from user mode to supervisor mode allows the CPU to have unrestricted access to these resources and instructions. This transition is typically initiated by an interrupt. Among the different types of interrupts, software interrupts are most appropriate for switching from CPU user mode to supervisor mode.

Explanation:
1. Internal interrupts:
Internal interrupts are generated by the CPU itself and are used for handling events such as divide by zero errors, page faults, or illegal instructions. While these interrupts can cause a switch from user mode to supervisor mode, they are not specifically designed for this purpose. Internal interrupts are more focused on handling errors or exceptional events within the CPU.

2. External interrupts:
External interrupts are generated by external devices, such as keyboard input, mouse input, or network events. These interrupts are also capable of switching the CPU from user mode to supervisor mode. However, like internal interrupts, their primary purpose is to handle events from external devices rather than specifically facilitating the mode transition.

3. Software interrupts:
Software interrupts, also known as system calls, are explicitly invoked by a software program to request a service from the operating system. These interrupts are designed to facilitate the transition from user mode to supervisor mode, allowing the program to access privileged instructions or resources. When a software interrupt is triggered, the CPU transfers control to a predefined interrupt handler routine in the operating system, which executes in supervisor mode. The interrupt handler then performs the requested service and returns control back to the user program.

Conclusion:
While internal and external interrupts can also trigger a switch from user mode to supervisor mode, software interrupts are specifically designed for this purpose. They provide a controlled mechanism for programs to request services from the operating system and access privileged instructions or resources. Therefore, software interrupts are the most appropriate type of interrupt for switching from CPU user mode to supervisor mode.