Explanation
A web application firewall (WAF) is a solution that inspects traffic to a cluster of web servers in a cloud environment and protects them from common web-based attacks, such as SQL injection, cross-site scripting, and denial-of-service1. A WAF can be deployed as a cloud service or as a virtual appliance in front of the web servers. A load balancer is a solution that distributes traffic among multiple web servers in a cloud environment and improves their performance, availability, and scalability2. A load balancer can also perform health checks on the web servers and route traffic only to the healthy ones. The other options are not relevant to this scenario. A CASB is a cloud access security broker, which is a solution that monitors and controls the use of cloud services by an organization's users3. A VPN is a virtual private network, which is a solution that creates a secure and encrypted connection between two networks or devices over the internet. TLS is Transport Layer Security, which is a protocol that provides encryption and authentication for data transmitted over a network. DAST is dynamic application security testing, which is a method of testing web applications for vulnerabilities by simulating attacks on them.
References:
1: https://www.imperva.com/learn/application-security/what-is-a-web-application-firewall-waf/
2:
https://www.imperva.com/learn/application-security/load-balancing/ 3:
https://www.imperva.com/learn/application-security/cloud-access-security-broker-casb/ :
https://www.imperva.com/learn/application-security/vpn-virtual-private-network/ :
https://www.imperva.com/learn/application-security/transport-layer-security-tls/ :
https://www.imperva.com/learn/application-security/dynamic-application-security-testing-dast/ :
https://docs.microsoft.com/en-us/azure/cloud-adoption-framework/ready/azure-best-practices/plan-for-traffic-ins
https://docs.microsoft.com/en-us/azure/private-link/inspect-traffic-with-azure-firewall :
https://docs.microsoft.com/en-us/azure/architecture/example-scenario/gateway/application-gateway-before-azure

MDM stands for Mobile Device Management, which is a software solution that can manage and secure smartphones, laptops, tablets and other mobile devices across heterogeneous platforms. MDM can enforce security features such as encryption, password policies, remote wipe, device tracking, app control and more.
MDM can also monitor and update the devices remotely and provide reports and alerts on their status. MDM is the most effective solution to implement security features across heterogeneous platforms, as it can provide centralized and consistent management of various types of devices. Verified References:
* Security+ (Plus) Certification | CompTIA IT Certifications
https://www.comptia.org/certifications/security (See Domain 3: Architecture and Design, Objective 3.4:
Given a scenario, implement secure systems design.)
* CompTIA Security+ 601 - Infosec
https://www.infosecinstitute.com/wp-content/uploads/2021/03/CompTIA-Security-eBook.pdf (See Security+: 5 in-demand cybersecurity skills, Implementation)
* Certification Security+ | CompTIA https://www.comptia.org/landing/securityplus/index.html (See Exam Objectives)