Introduction:
System calls are an integral part of the operating system. They allow user programs to request services from the operating system kernel. These services may include tasks such as file operations, process management, memory allocation, and input-output operations. In order to invoke a system call, specific instructions need to be executed.

Explanation:
System calls are usually invoked using a software interrupt. A software interrupt is a mechanism that allows a program to transfer control to a specific routine (known as an interrupt handler) in the operating system kernel.

Privileged Instruction:
A privileged instruction is a type of instruction that can only be executed in a privileged mode, such as kernel mode. These instructions are restricted to the operating system kernel and are not accessible to user programs. Since system calls are meant to be accessible to user programs, they cannot be invoked using privileged instructions.

Indirect Jump:
An indirect jump is a type of instruction that transfers control to an address specified by a register or memory location. While indirect jumps can be used to invoke system calls, they are not the usual method. Indirect jumps are more commonly used in other contexts, such as function pointers or dynamic dispatch.

Polling:
Polling is a technique where a program repeatedly checks the status of a resource, such as a hardware device, in order to determine if it is ready for use. While polling can be used to wait for the completion of a system call, it is not the method used to actually invoke the system call itself.

Software Interrupt:
The correct answer is option C: a software interrupt. A software interrupt is a specific type of interrupt that is generated by a software instruction. In the case of system calls, a software interrupt instruction is used to transfer control to a specific interrupt handler in the operating system kernel. This interrupt handler is responsible for executing the requested system call and returning the result to the user program.

Conclusion:
System calls are usually invoked using a software interrupt. This allows user programs to request services from the operating system kernel. While other methods, such as privileged instructions, indirect jumps, or polling, can be used in certain contexts, they are not the usual method for invoking system calls.