Explanation
Anti-affinity rules are rules or policies that prevent two or more VMs from running on the same host or cluster in a cloud environment. Anti-affinity rules can provide high availability to an application environment in case of hypervisor failure, as they can distribute or separate the application VMs across different hosts or clusters and avoid having a single point of failure. Anti-affinity rules can also improve performance and reliability, as they can reduce contention and load by balancing the resource utilization across multiple hosts or clusters.
References: CompTIA Cloud+ Certification Exam Objectives, page 10, section 1.5

Explanation
Regression testing is a type of testing that ensures that new code or changes to existing code do not break or degrade the functionality of the software. Regression testing is often used in software development workflows to verify that new features or bug fixes do not introduce new errors or affect the performance of the software.
Regression testing can help prevent negative impacts on existing automation activities by checking that the new code is compatible with the existing code and does not cause any unexpected failures or errors.
References: CompTIA Cloud+ Certification Exam Objectives, page 19, section 4.1

Explanation
Database replication is the best technique to migrate databases from an on-premises IaaS-hosted environment to a public DBaaS solution. Database replication is a process of copying data from one database server to another database server in real-time or near real-time. Database replication can ensure data consistency and availability across different locations and platforms. Database replication can facilitate migration by synchronizing data between on-premises databases and cloud databases.

Explanation
According to the CompTIA Cloud+ Study Guide1, autoscaling is "the ability to automatically increase or decrease the number of resources allocated to a cloud service based on the current demand". This means that autoscaling can help a cloud environment adjust to changes in traffic and workload, such as the ones caused by the new marketing promotions.
Ballooning, on the other hand, is "a technique used by hypervisors to reclaim unused memory from a virtual machine and allocate it to another virtual machine that needs more memory". This means that ballooning can help optimize the memory usage of a cloud environment, but it does not affect the number of resources allocated to a cloud service.
A firewall is "a device or software that monitors and controls the incoming and outgoing network traffic based on predefined rules". This means that a firewall can help protect a cloud environment from unauthorized access and malicious attacks, but it does not affect the number of resources allocated to a cloud service.
Switches are "devices that connect multiple devices on a network and forward data packets between them".
This means that switches can help improve the network performance and connectivity of a cloud environment, but they do not affect the number of resources allocated to a cloud service.
Based on this information, I think the best answer to your question is D. Autoscaling. Autoscaling can help the company accommodate the new traffic by automatically increasing or decreasing the number of resources allocated to their web-application service based on the current demand. This can also help reduce costs and improve performance and availability.
I hope this helps you understand the concept of autoscaling better. If you want to learn more about CompTIA Cloud+, you can check out some of these resources:
CompTIA Cloud+ : Cloud High Availability & Scaling: A video course that covers the topics of high availability and scaling in cloud environments, including autoscaling, horizontal scaling, vertical scaling and cloud bursting.
Cloud+ (Plus) Certification | CompTIA IT Certifications: The official website of CompTIA Cloud+, where you can find exam details, preparation materials, renewal information and more.

Explanation
Simultaneous multithreading (SMT) is a type of CPU technology that allows multiple threads to run concurrently on a single CPU core. Enabling SMT can help achieve the goal of having the VMs on the hypervisor share CPU resources on the same core when feasible, as it can increase the CPU utilization and efficiency by executing more instructions per cycle and reducing idle time or wasted cycles. Enabling SMT can also improve performance and throughput, as it can speed up processing and handle increased workload or demand. References: CompTIA Cloud+ Certification Exam Objectives, page 9, section 1.4