Step 1: Use an everyday analogy.
Imagine a worker who is interrupted mid task, carefully notes down exactly where they stopped, then picks up a completely different task, and later returns to the first one right where they left off. The operating system does something very similar with processes on a single CPU.
Step 2: Translate the analogy into OS terms.
When the OS decides to stop running one process and start another, it saves the current process's state, registers, program counter, and other details, into its PCB, then loads the saved state of the next process from its PCB into the CPU. This save and load cycle is what allows one physical CPU to appear to run many processes at once.
Step 3: Eliminate the unrelated options.
Switching between networks concerns networking hardware, not the CPU. Changing the user interface mode is a display concept, and allocating additional memory is a memory management activity, none of these describe what happens when the CPU itself changes which process it executes.
Step 4: State the definition.
This save and resume mechanism between processes is exactly
\[ \boxed{\text{Switching the CPU from one process to another}} \]