Explanation
The command if [ -f "$filename" ]; then checks if the variable $filename refers to a regular file that exists.
The -f option is used to test for files. If the condition is true, the commands after then are executed. This is the correct way to replace the <CONDITIONAL> string. The other options are incorrect because they either use the wrong option (-d tests for directories), the wrong syntax (missing a semicolon after the condition), or the wrong keyword (while is used for loops, not conditions). References: CompTIA Linux+ (XK0-005) Certification Study Guide, Chapter 16: Writing and Executing Bash Shell Scripts, page 493.

Explanation
The command that will help the administrator install GRUB on the legacy MBR of the SATA hard drive is grub-install /dev/sda. This command will install GRUB on the master boot record (MBR) of the first SATA disk (/dev/sda). The MBR is the first sector of a disk that contains boot code and a partition table. GRUB will overwrite the boot code and place its own code that can load GRUB modules and configuration files from a specific partition.
The other options are not correct commands for installing GRUB on the legacy MBR of the SATA hard drive.
The grub-install /dev/hda command will try to install GRUB on the first IDE disk (/dev/hda), which may not exist or may not be bootable. The grub-install /dev/sr0 command will try to install GRUB on the first SCSI CD-ROM device (/dev/sr0), which is not a hard drive and may not be bootable. The grub-install /dev/hd0,0 command is invalid because grub-install does not accept partition names as arguments, only disk names. References: Installing GRUB using grub-install; GRUB Manual

The command that should be run on the DNS forwarder server to accomplish the task is firewall-cmd --zone=public --add-port=53/udp --permanent. The firewall-cmd command is a tool for managing firewalld, which is a firewall service that provides dynamic and persistent network security on Linux systems. The firewalld uses zones and services to define the rules and policies for the network traffic. The zones are logical groups of network interfaces and sources that have the same level of trust and security. The services are predefined sets of ports and protocols that are associated with certain applications or functions. The --zone=public option specifies the zone name that the rule applies to. The public zone is the default zone that represents the untrusted network, such as the internet. The --add-port=53/udp option adds a port and protocol to the zone. The 53 is the port number that is used by the DNS service. The udp is the protocol that is used by the DNS service. The --permanent option makes the change persistent across reboots. The command firewall-cmd --zone=public --add-port=53/udp --permanent will modify the firewall on the server for the DNS forwarder to allow the internal servers to communicate to it and make the changes persistent between server reboots. This is the correct command to use to accomplish the task. The other options are incorrect because they either do not modify the firewall on the server for the DNS forwarder (ufw allow out dns or systemctl reload firewalld) or do not use the correct syntax for the command (iptables -A OUTPUT -p udp -ra udp -dport 53 -j ACCEPT instead of iptables -A OUTPUT -p udp -ra udp --dport 53 -j ACCEPT). Reference: CompTIA Linux+ (XK0-005) Certification Study Guide, Chapter 12: Managing Network Connections, page 392.

The docker exec <container_name> ls command can be used to verify whether the built container has the app.go file in its root directory. This command will run the ls command inside the specified container and list the files and directories in its root directory. If the app.go file is present, it will be displayed in the output. The docker image inspect command will display information about an image, not a container, and it will not list the files inside the image. The docker container inspect command will display information about a container, not its files. The docker ps <container_name> command is invalid, as ps does not accept a container name as an argument. Reference: CompTIA Linux+ (XK0-005) Certification Study Guide, Chapter 16: Virtualization and Cloud Technologies, page 499.

Explanation
The chattr command is used to change file attributes on Linux systems that support extended attributes, such as ext2, ext3, ext4, btrfs, xfs, and others. File attributes are flags that modify the behavior of files and directories.
To prevent a file from being modified by all users in a Linux system, including the root account, the development team can use the chattr +i /app/conf/file command. This command will set the immutable attribute (+i) on the file /app/conf/file, which means that the file cannot be deleted, renamed, linked, appended, or written to by any user or process. To remove the immutable attribute, the development team can use the chattr -i /app/conf/file command. The statement C is correct.
The statements A, B, and D are incorrect because they do not prevent the file from being modified by all users.
The chmod /app/conf/file command does not work because it requires an argument to specify the permissions to change. The setenforce /app/conf/file command does not work because it is used to change the SELinux mode, not file attributes. The chmod 0000 /app/conf/file command will remove all permissions from the file, but it can still be modified by the root account. References: [How to Use chattr Command in Linux]