DLP stands for Data Loss Prevention and it is a system that scans outgoing email for account numbers, sensitive phrases, and other forms of PII (Personally Identifiable Information). DLP can help prevent data breaches, comply with regulations, and protect the privacy of customers and employees. DLP can also block, encrypt, or quarantine emails that contain sensitive data. References:
https://www.comptia.org/training/resources/exam-objectives/comptia-server-sk0-005-exam-objectives (Objective 3.2)

A differential backup is a type of backup that copies all the files that have changed since the last full backup.
A differential backup requires more storage space than an incremental backup, which only copies the files that have changed since the last backup of any type, but it also requires less time to restore in case of data loss. By combining a full, weekly backup with daily differential backups, the administrator can ensure that only two backup sets are needed to restore a full backup as quickly as possible. Verified References: [Incremental vs Differential Backup]

The best course of action for the company is to upgrade the application package to fix the known vulnerability.
A vulnerability is a weakness or flaw in an application that can be exploited by an attacker to compromise the security or functionality of the system. Upgrading the application package means installing a newer version of the application that has patched or resolved the vulnerability. This way, the company can prevent potential attacks that may exploit the vulnerability and cause damage or loss.

Grandfather-father-son (GFS) is a common backup rotation scheme that uses daily, weekly, and monthly backup media in a first-in, first-out (FIFO) method. The daily backups are rotated on a 3-months basis using a FIFO system as above. The weekly backups are similarly rotated on a bi-yearly basis, and the monthly backups are rotated on an annual basis. The oldest backup media in each cycle are overwritten by the newest ones. This scheme provides multiple versions of backup data at different intervals, allowing for flexible restoration options. Waterfall is another name for GFS. Synthetic full is a backup method that combines an initial full backup with subsequent incremental backups to create a new full backup without transferring all data again. Tower of Hanoi is another backup rotation scheme that uses an algorithm based on moving disks between three pegs. References:
* https://en.wikipedia.org/wiki/Backup_rotation_scheme

A DNS entry is a record that maps a domain name to an IP address. If the DNS entry for the new application is incorrect, users will not be able to resolve the domain name to the correct IP address and port number. This will prevent them from accessing the application, even if the firewall rules allow port 8443. To fix this issue, the server administrator should verify and update the DNS entry for the new application.
References: CompTIA Server+ Study Guide, Chapter 6: Networking, page 230.