Explanation
The excerpt C is a valid if condition in the Bourne shell because it uses the correct syntax and operators for the test command. The syntax of the if statement in the Bourne shell is:
if test-commands; then consequent-commands; [elif more-test-commands; then more-consequents;] [else alternate-consequents;] fi The test-commands are any commands that return a status of 0 (true) or non-zero (false). The test command is a common way to perform various tests on files, strings, and numbers. The test command has the following format:
test expression
The expression consists of one or more primaries, which are unary or binary operators that evaluate to true or false. The primaries can be combined with the following operators:
! expression: Logical NOT expression -a expression: Logical AND expression -o expression: Logical OR ( expression ): Parentheses for grouping The excerpt C uses the test command with the following expression:
"$var" = "test"
This expression tests if the value of the variable $var is equal to the string "test". The = operator is a string comparison operator that returns true if the strings are equal. The double quotes around the variable and the string are used to prevent word splitting and globbing by the shell. The spaces around the operator are also required by the test command.
The excerpt C also uses the correct syntax for the then and fi keywords, which mark the beginning and the end of the if statement. The then keyword must be on the same line as the test command, or on a new line after a semicolon. The fi keyword must be on a new line.
The other excerpts are invalid for various reasons:
The excerpt A uses the && operator instead of the -a operator for logical AND. The && operator is a shell operator that executes the second command only if the first command succeeds. The test command does not recognize the && operator as a valid argument. The excerpt A also uses the != operator instead of the -ne operator for numeric comparison. The != operator is a string comparison operator that returns true if the strings are not equal. The -ne operator is a numeric comparison operator that returns true if the numbers are not equal.
The excerpt B uses the [ [ construct instead of the test command. The [ [ construct is a shell keyword that performs conditional expressions similar to the test command, but with some extensions and differences. The [ [ construct is not part of the POSIX standard and is not supported by all shells. The Bourne shell does not support the [ [ construct, and will treat it as a command name. The excerpt B also uses the wrong syntax for the then keyword, which must be on a new line or after a semicolon.
The excerpt D uses the wrong syntax for the test command, which requires a space after the opening bracket and before the closing bracket. The excerpt D also uses the wrong syntax for the fi keyword, which must be on a new line.

Explanation
The zfs snapshot command creates a snapshot of a ZFS dataset, which is a point-in-time copy of the data. The snapshot name is specified as follows: filesystem@snapname or volume@snapname. The snapshot name must satisfy the naming requirements in ZFS Component Naming Requirements1. In this case, the command zfs snapshot space/data@snap1 creates a snapshot of the dataset space/data named snap1. The other options are either invalid or do not create a snapshot. References: 1:https://docs.oracle.com/cd/E19253-01/819-5461/gbcya/index.html

Explanation
The zpool create command is used to initialize ZFS on one or more block devices and create a storage pool. A storage pool is a collection of devices that provide physical storage and data replication for ZFS datasets. The zpool create command takes the name of the pool and the devices to be added as arguments. For example, zpool create mypool c0t0d0 c0t1d0 c0t2d0 would create a pool named mypool with three devices. The other options are either invalid or do not initialize ZFS. References: 1:
https://www.comptia.org/training/books/a-core-1-220-1101-study-guide, page 302 2:
https://docs.oracle.com/cd/E19253-01/819-5461/gbchx/index.html, section "Creating a ZFS Storage Pool"

Explanation
This command and option can be used to perform a reverse DNS lookup, which is the process of finding the domain name or hostname associated with an IP address. The dig command is a tool that can query DNS servers for various types of records, such as A, MX, NS, PTR, etc. The -x option tells dig to perform a reverse DNS lookup by sending a PTR query to the DNS server. For example, to perform a reverse DNS lookup for the IP address 8.8.8.8, the following command can be used:
dig -x 8.8.8.8
The output of this command will show the PTR record for 8.8.8.8, which is dns.google. This means that the hostname for 8.8.8.8 is dns.google. Reverse DNS lookups are useful for verifying the identity of a host, checking for mail server configuration, or troubleshooting network problems

Explanation
The cat command is used to concatenate and display files. The mail command is used to send and receive mail messages. The pipe symbol (I) is used to redirect the output of one command to the input of another command.
Therefore, the command cat myconfigfile I mail [email protected] sends the contents of the file myconfigfile by email to [email protected]. The cat command reads the file and displays its contents to the standard output, which is then piped to the mail command, which reads the standard input and sends it as a mail message to the specified address. The mail command also prompts the user to enter a subject line and optionally some additional headers for the message. Alternatively, the subject line can be specified by the -s option of the mail command. For example, the command cat myconfigfile I mail -s "My config file" [email protected] sends the same message with the subject "My config file". References:
[cat - FreeBSD], the manual page for the cat command on FreeBSD.
[mail - FreeBSD], the manual page for the mail command on FreeBSD.
[How to send email from the Linux command line - LinuxConfig.org], a tutorial on how to use the mail command and other email clients on Linux.