Time taken to switch two processes is very large as compared to time taken to switch between kernel and user mode of execution because :
When you switch processes, you have to do a context switch, save the PCB of previous process (note that the PCB of a process in Linux has over $95$ entries), then save registers and then load the PCB of new process and load its registers etc.
When you switch between kernel and user mode of execution, OS has to just change a single bit at hardware level which is very fast operation.
So, answer is: (C).