Explanation
Distributed denial-of-service (DDoS) protection is a type of security solution that detects and mitigates DDoS attacks that aim to overwhelm or disrupt a system or service by sending large volumes of traffic from multiple sources. DDoS protection can prevent such disruptive traffic from reaching the web servers by filtering out malicious or unwanted traffic and allowing only legitimate traffic to pass through. DDoS protection can also help maintain the availability and functionality of web services and applications during a DDoS attack.
References: CompTIA Cloud+ Certification Exam Objectives, page 14, section 2.7

Explanation
The best option to implement a new three-host cluster with a limited budget for testing purposes is to use three public cloud hosts with six cores, 80GB of RAM, 180GB of storage, and 150Mbps of bandwidth each. This option will provide enough resources to run all the applications without exceeding their estimated consumption, while also minimizing the cost and complexity of the cluster. The other options either provide insufficient or excessive resources, which could affect the performance or cost of the cluster. Reference:
[CompTIA Cloud+ Certification Exam Objectives], Domain 1.0 Configuration and Deployment, Objective 1.2 Given a scenario involving requirements for deploying an application in the cloud, select an appropriate solution design.

A virtual application delivery controller (vADC) is a type of network device or software that provides load balancing, security, and optimization for web applications or services. Deploying a vADC appliance can help prevent an outage of the organization's web server infrastructure due to a spike in network traffic, as it can distribute the traffic across multiple web servers and improve the performance and availability of web applications or services. Deploying a vADC appliance can also provide mitigation methods such as DDoS protection, SSL offloading, and caching to enhance the security and efficiency of web traffic delivery.
References: CompTIA Cloud+ Certification Exam Objectives, page 15, section 2.8

An incident response plan is a document or procedure that defines the roles, responsibilities, and actions to be taken in the event of a security incident or breach. Having a detailed incident response plan can help mitigate the risk of a zero-day vulnerability most efficiently, as it can provide a clear and consistent framework for identifying, containing, analyzing, and resolving any potential threats or exploits related to the unknown or unpatched vulnerability. Having a detailed incident response plan can also help minimize the impact and damage of a security incident or breach, as it can enable timely and effective recovery and restoration processes. Reference: CompTIA Cloud+ Certification Exam Objectives, page 14, section 2.7