Operating System - Trap and Interrupts

Question

Which of the following statements is/are true regarding how the trap instruction (e.g., int n in x86) is invoked when a trap occurs in a system? 

1. When a user makes a system call, the trap instruction is invoked by the kernel code handling the system call 

2. When a user makes a system call, the trap instruction is invoked by userspace code (e.g., user program or a library) 
3. When an external I/O device raises an interrupt, the trap instruction is invoked by the device driver handling the interrupt
4. When an external I/O device raises an interrupt signaling the completion of an I/O request, the trap instruction is invoked by the user process that raised the I/O request