What is the first program that is usually started, at boot time, by the Linux kernel when using SysV init?

/lib/init.so is a fabricated path - the init process is a standalone executable, not a shared library loaded from /lib.

/sbin/init is the conventional path for the SysV init binary and is the first userspace process spawned by the kernel at boot, becoming PID 1. The kernel locates it via a compiled-in default path or the init= kernel parameter, and it then reads /etc/inittab to determine which runlevel to enter and which services to start.

/etc/rc.d/rcinit does not exist as a standard path; /etc/rc.d contains runlevel service scripts that are invoked by init, not init itself.

/proc/sys/kernel/init is a kernel parameter tunable that can store the init path but is not itself an executable program that the kernel runs.

/boot/init is not a standard location for the init binary; /boot holds the kernel image and bootloader files, not userspace executables like init.