"Why is the process state (i. e., PC, SP, EFLAGS, general registers) kept in the Kernel Interrupt Stack before handling an Interrupt? Why could we not store it in the user memory? What is the risk?"

We couldn't have the process condition or state throughout the client storage for the following reasons that are given below.

Explanation:

OS simply can not trust client processes because it could be unreliable or harmful to either the user.If the user is crashing it shouldn't impact the operating system.When many tasks are functioning at the moment then complete storage use will occur in a certain situation the OS should also have remaining memory to execute valuable directions.The OS wants to be equal to all application processes, therefore it requires to provide its resources towards that.

So, it's the right answer.

That display data that change over time