Explanation
The first program that is usually started, at boot time, by the Linux kernel when using SysV init is /sbin/init.
This program is responsible for reading the /etc/inittab file and executing the appropriate scripts and programs for each runlevel. The other options are not valid programs that are started by the kernel. /lib/init.so is a shared library that is used by some init programs, but not by SysV init. /etc/rc.d/rcinit is a script that is run by init, not by the kernel. /proc/sys/kernel/init is a kernel parameter that can be used to specify a different init program, but the default value is /sbin/init. /boot/init is not a standard location for an init program, and it is unlikely that the kernel would find it there. References:
* 1: SysVinit - ArchWiki
* 2: Linux: How to write a System V init script to start, stop, and restart my own application or service - nixCraft
* 3: sysvinit - Gentoo wiki