Deploying web servers into an IaaS (Infrastructure as a Service) provider is the most suitable method to achieve the objective of hosting an external website and managing the OS. IaaS is a cloud service model that provides basic computing resources such as servers, storage, network, etc., to the customers. The customers have full control and flexibility over these resources and can install and configure any software they need on them. IaaS is suitable for hosting web servers and managing the OS, as it allows the customers to choose their preferred OS, web server software, settings, etc., and customize them according to their needs.

Tags are metadata or labels that can be attached to cloud resources, such as VMs, storage, or networks. Tags can help organize, identify, and manage cloud resources, as well as track their usage and costs. Tags can also be used to implement chargeback or showback policies, which are methods of allocating the cloud services bill to different departments or consumers based on their consumption of resources .
A cloud administrator can modify the tags for all the unmapped resources to correct the billing issue. By adding or updating the tags with the relevant department or consumer name, the administrator can ensure that the resources are billed to the correct entity. The administrator can also use the tags to filter, sort, or group the resources on the billing dashboard, and generate reports or alerts based on the tags .
Checking the utilization of the resources may help identify the purpose or owner of the resources, but it will not help correct the billing issue. The administrator still needs to modify the tags for the resources to assign them to the appropriate department or consumer.
Modifying the chargeback details of the consumer may help adjust the billing rate or method for a specific consumer, but it will not help correct the billing issue. The administrator still needs to modify the tags for the resources to associate them with the consumer.
Adding the resources to the consumer monitoring group may help monitor the performance or availability of the resources for a specific consumer, but it will not help correct the billing issue. The administrator still needs to modify the tags for the resources to link them with the consumer.

A server cluster is a group of servers that work together to provide high availability, load balancing, and scalability for applications or services. A server cluster can help ensure the high availability requirement for migrating an e-commerce company's main website to a cloud provider, as it can prevent downtime or disruption in case of a server failure or outage by automatically switching the workload to another server in the cluster. A server cluster can also improve performance and reliability, as it can distribute the workload across multiple servers and handle increased traffic or demand. Reference: CompTIA Cloud+ Certification Exam Objectives, page 10, section 1.5

Backup as a service (BaaS) is the best approach for changing the backup strategy to minimize manual intervention after a data loss due to an on-premises file-server crash. BaaS is a cloud-based service that provides backup and recovery solutions for customers' data and systems. BaaS can automate and simplify backup processes by using cloud storage, encryption, deduplication, compression, scheduling, etc., without requiring customers to purchase or maintain backup hardware or software.

To conclude the performance test of a public application, the organization needs to verify the application works well under an unexpected volume of requests. This is also known as a stress test, which is a type of performance test that evaluates the behavior and stability of the application under extreme conditions1. A stress test can help identify potential bottlenecks, errors, or failures that may occur when the application is subjected to a sudden surge or spike in demand2. A stress test can also help determine the maximum capacity and scalability of the application3.