Explanation
To update FreeBSD, the freebsd-update fetch command is used to download available binary updates, and the freebsd-update install command is used to apply the downloaded updates to the system. These commands are part of the freebsd-update utility, which supports binary security and errata updates to the FreeBSD base system without the need for manual compilation and installation2.
References:
FreeBSD Handbook - freebsd-update

mkdir
Explanation
The command mkdir (short for make directory) creates a new directory with the specified name. The command takes one or more arguments, which are the names of the directories to be created. If the name contains spaces or other special characters, it should be enclosed in quotes. For example, mkdir "My Documents" creates a directory named My Documents. The command also accepts some options, such as -p, which creates the parent directories if they do not exist, or -m, which sets the permissions of the new directory.
For example, mkdir -p -m 755 /usr/local/bin creates the directory /usr/local/bin and its parent directories /usr and /usr/local, and sets the permissions of /usr/local/bin to 755 (read, write and execute for the owner, read and execute for the group and others). References:
[mkdir(1) - FreeBSD Manual Pages]
[How to Create a Directory in Linux - Linuxize]

Explanation
The command cp SI 52 in the Bourne shell script copies the file named by the value of SI to the file named 52.
The SI is a positional parameter that represents the first argument passed to the script when it is executed.
Positional parameters are numbered from 1 to 9, and can be accessed by using the dollar sign $ followed by the number. For example, $1 is the first positional parameter, $2 is the second, and so on. Therefore, the SI in the script is equivalent to $1, and it represents the first argument passed to the script. The correct answer is D.
A positional parameter. References:
[Positional Parameters - FreeBSD]
[Shell Scripting Tutorial - Positional Parameters]

Explanation
The name of the NetBSD installation program is sysinst. It is the standard installer used by NetBSD for setting up the system on a new machine. The sysinst program guides users through the installation process, including disk partitioning, package selection, and system configuration45.
References:
NetBSD Guide - Example installation
NetBSD Guide - Installing NetBSD: Preliminary considerations and preparations