A digital signature is a cryptographic technique that uses the sender's private key to generate a unique code for a message or document. The receiver can use the sender's public key to verify the authenticity and integrity of the message or document. A digital signature can prevent unauthorized change, as any modification to the message or document will invalidate the signature and alert the receiver of tampering.
References
What is a digital signature?
Digital Signature - an overview | ScienceDirect Topics
ISACA CISA Review Manual, 27th Edition, page 253

A web-based CRM system that is directly accessed by customers via the Internet should be hosted in a secure and isolated environment to protect it from external threats and unauthorized access. A web-based CRM system should also be reliable, trusted, and backedup regularly1.
Hosting the system on an external third-party service provider's servers (A) or a hybrid-cloud platform managed by a service provider (B) may not be a concern for the auditor if the service provider has adequate security measures and service level agreements in place. The auditor should verify the security controls and contractual terms of the service provider before trusting them with the CRM data23.
Hosting the system within a demilitarized zone (DMZ) of a corporate network is a common practice to provide an extra layer of security to the CRM system from untrusted networks, such as the Internet. A DMZ is a perimeter network that isolates the CRM system from the internal network and filters the incoming traffic from the external network using a security gateway4567.
Hosting the system within an internal segment of a corporate network (D) is a concern for the auditor because it exposes the CRM system and the internal network to potential attacks from the Internet. The CRM system should not be directly accessible from the Internet without a DMZ or a firewall to protect it. This could compromise the confidentiality, integrity, and availability of the CRM data and the internal network78.

Data classification is the first consideration when deciding whether data should be moved to a cloud provider for storage because it determines the level of protection and security required for the data. Data classification also helps to identify the legal and regulatory requirements that apply to the data, such as privacy, retention and disposal policies. Data storage costs, vendor cloud certification and service level agreements (SLAs) are important factors to consider, but they are secondary to data classification. References: CISAReview Manual (Digital Version) 1, Chapter 5, Section 5.3.2

Risk-based auditing is an audit approach that focuses on the analysis and management of risk within an organization. Risk-based auditing helps identify and prioritize the areas or processes that pose the highest risk to the organization's objectives and allocate audit resources accordingly. Risk-based auditing also helps provide assurance and advisory services related to the organization's risk management processes and controls.
By using risk-based auditing, internal auditors can optimize the use of their audit resources and add value to the organization.
Agile auditing, continuous auditing, and outsourced auditing are not audit approaches that are most helpful in optimizing the use of IS audit resources. Agile auditing is a flexible and iterative audit methodology that adapts to changing circumstances and stakeholder needs. Continuous auditing is a method of performing audit activities on a real-time or near-real-time basis using automated tools and techniques. Outsourced auditing is a practice of contracting external auditors to perform some or all of the internal audit functions. These audit methods may have some advantages or disadvantages depending on the context and objectives of the audit, but they do not necessarily optimize the use of IS audit resources.