Which program is the first one started at boot time by the Linux kernel?

The correct answer is the program located at /sbin/init. This program is pivotal because it is the first user-space program that the Linux kernel executes after it has completed the booting process. Once the kernel has initialized all system hardware and has effectively set up its environment, it hands control over to the init process.

The init program is responsible for spooling up all other necessary processes and services that your system requires to function properly. It plays a crucial role in managing the desired run level (or target), which specifies what services and processes should be started during system boot. The init program can spawn other processes, maintain their states, and respond to various system signals, making it central to system management.

The other options listed, like /bin/bash (a shell), /usr/sbin/httpd (the Apache web server), and /usr/bin/sshd (the SSH daemon), are not executed at boot time by the kernel directly. They are typically started by init or other service management systems as required once the boot process is well underway. Each of these programs serves distinct roles in the Linux environment but does not assume the initial role that init holds in the boot sequence.