Let the time taken to switch from user mode to kernel mode of execution be $T1$ while time taken to switch between two user processes be $T2$. Which of the following is correct?
Notice important point mentioned by @Sachin Mittal 1 -> Context switches can occur only in kernel mode.
This Link might be useful for understanding.
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).
Yes Arjun sir, you are correct,
honestly saying, I didnt even read your comment before writing the above comment.
Context switching can be described in slightly more detail as the kernel (i.e., the core of the operating system) performing the following activities with regard to processes (including threads) on the CPU: (1) suspending the progression of one process and storing the CPU's state (i.e., the context) for that process somewhere in memory, (2) retrieving the context of the next process from memory and restoring it in the CPU's registers and (3) returning to the location indicated by the program counter (i.e., returning to the line of code at which the process was interrupted) in order to resume the process.
Context switches can occur only in kernel mode. Kernel mode is a privileged mode of the CPU in which only the kernel runs and which provides access to all memory locations and all other system resources. Other programs, including applications, initially operate in user mode, but they can run portions of the kernel code via system calls.