The best recommendation to prevent unauthorized access to cloud-based applications and data is to implement multi-factor authentication (MFA). MFA is a method of verifying the identity of a user by requiring two or more pieces of evidence, such as a password, a code sent to a phone, or a biometric factor. MFA adds an extra layer of security to prevent unauthorized access, even if the user's password is compromised or stolen. MFA can also help comply with data privacy and security regulations, such as the General Data Protection Regulation (GDPR) and the Health Insurance Portability and Accountability Act (HIPAA).
The other options are not as effective as MFA in preventing unauthorized access. An intrusion detection system (IDS) is a tool that monitors network traffic and alerts administrators of suspicious or malicious activity, but it does not prevent access by itself. Updating security policies and procedures is a good practice, but it does not ensure that users follow them or that they are enforced. Utilizing strong anti-malware controls on all computing devices can help protect against malware infections, but it does not prevent users from accessing cloud-based applications and data from any Internet-connected web browser.
References:
* ISACA, CISA Review Manual, 27th Edition, 2019, p. 2471
* ISACA, CISA Review Questions, Answers & Explanations Database - 12 Month Subscription2
* What Is Cloud Security? | Google Cloud3
* 5 Cloud Application Security Best Practices | Snyk4

Configuring each authentication server as belonging to a cluster of authentication servers is the best way to minimize performance degradation of servers used to authenticate users of an e-commerce website. A cluster is a group of servers that work together to provide high availability, load balancing, and fault tolerance. If one server fails or becomes overloaded, another server in the cluster can take over its workload without disrupting the service. A single server as a primary authentication server and a second server as a secondary authentication server is not as effective as a cluster, because the secondary server is only used when the primary server fails, which means it is idle most of the time and does not improve performance. Configuring each authentication server and ensuring that each disk of its RAID is attached to the primary controller does not address the issue of performance degradation, but rather the issue of data redundancy and reliability.
RAID (redundant array of independent disks) is a technology that combines multiple disks into a logical unit that can tolerate disk failures and improve data access speed. Configuring each authentication server and ensuring that the disks of each server form part of a duplex does not address the issue of performance degradation, but rather the issue of data backup and recovery. A duplex is a pair of disks that store identical copies of data, so that if one disk fails, the other disk can be used to restore the data. References: ISACA CISA Review Manual 27th Edition, page 310