Explanation:

System calls are a way for user-level programs to request services from the operating system. These services can include tasks such as reading or writing files, creating processes, allocating memory, and many others. In order to invoke a system call, a program needs to switch from user mode to kernel mode, where the operating system resides.

There are several ways to invoke a system call, but the most common methods are using a software interrupt and polling.

1. Software Interrupt:
A software interrupt is a mechanism provided by the processor to allow user-level programs to request services from the operating system. When a program wants to invoke a system call, it triggers a software interrupt instruction, which transfers control to a specific interrupt handler in the operating system. This interrupt handler then determines the requested service and performs the necessary actions. The software interrupt method is widely used because it provides a simple and efficient way to switch from user mode to kernel mode.

2. Polling:
Polling is another method that can be used to invoke system calls. In this method, the program continuously checks a certain status flag or register to see if a specific condition has been met. When the condition is met, the program knows that the system call has been completed and can proceed with the next instruction. Polling is often used in situations where the program needs to wait for a specific event to occur, such as the completion of an I/O operation.

Indirect Jump and Privileged Instruction:
Option 1, which mentions indirect jump, is not a method commonly used to invoke system calls. Indirect jumps are typically used for branching within a program, rather than for switching between user mode and kernel mode.

Option 4, which mentions privileged instruction, is also not a method used to invoke system calls. Privileged instructions are instructions that can only be executed in kernel mode, and they are typically used by the operating system itself, rather than by user-level programs.

Conclusion:
Therefore, the correct answer is option 'A', which includes software interrupt (option 2) and polling (option 3) as the commonly used methods to invoke system calls.