A Cloud Access Security Broker (CASB) solution is the most suitable option for securing an organization that has adopted a cloud-first strategy and does not have an on-premises IT infrastructure. CASBs provide visibility and control over shadow IT services, enforce security policies, and protect data across cloud services.
References = CompTIA Security+ SY0-701 study materials, particularly in the domain of cloud security and managing risks associated with shadow IT.

Explanation
The correct answer is D because it allows only the device with the IP address 10.50.10.25 to send outbound DNS requests on port 53, and denies all other devices from doing so. The other options are incorrect because they either allow all devices to send outbound DNS requests (A and C), or they allow no devices to send outbound DNS requests (B). References = You can learn more about firewall ACLs and DNS in the following resources:
* CompTIA Security+ SY0-701 Certification Study Guide, Chapter 4: Network Security1
* Professor Messer's CompTIA SY0-701 Security+ Training Course, Section 3.2: Firewall Rules2
* TOTAL: CompTIA Security+ Cert (SY0-701) | Udemy, Section 6: Network Security, Lecture 28:
Firewall Rules3

Changing the default password for the local administrator account on a VPN appliance is a basic security measure that would have most likely prevented the unexpected login to the remote management interface. Default passwords are often easy to guess or publicly available, and attackers can use them to gain unauthorized access to devices and systems. Changing the default password to a strong and unique one reduces the risk of brute-force attacks and credential theft. Using least privilege, assigning individual user IDs, and reviewing logs more frequently are also good security practices, but they are not as effective as changing the default password in preventing the unexpected login.

Detailed Explanation:
A tabletop exercise is a discussion-based activity where stakeholders simulate a security breach scenario to identify gaps in response plans and clarify roles and responsibilities. Reference: CompTIA Security+ SY0-
701 Study Guide, Domain 5: Security Program Management, Section: "Incident Response Planning and Exercises".

A vulnerability scan is the most likely method to identify legacy systems. These scans assess an organization's network and systems for known vulnerabilities, including outdated or unsupported software (i.e., legacy systems) that may pose a security risk. The scan results can highlight systems that are no longer receiving updates, helping IT teams address these risks.
Bug bounty programs are used to incentivize external researchers to find security flaws, but they are less effective at identifying legacy systems.
Package monitoring tracks installed software packages for updates or issues but is not as comprehensive for identifying legacy systems.
Dynamic analysis is typically used for testing applications during runtime to find vulnerabilities, but not for identifying legacy systems.