The greatest risk associated with storing customer data on a web server is data confidentiality. Data confidentiality is the property that ensures that data are accessible only to authorized entities or individuals, and protected from unauthorized disclosure or exposure. Storing customer data on a web server poses a high risk to data confidentiality, as web servers are exposed to the internet and may be vulnerable to various types of attacks or breaches that can compromise the security and privacy of customer data, such as hacking, phishing, malware, denial of service (DoS), etc. Customer data may contain sensitive or personal information that can cause harm or damage to customers or the organization if disclosed or exposed, such as identity theft, fraud, reputation loss, legal liability, etc. Data availability is the property that ensures that data are accessible and usable by authorized entities or individuals when needed. Data availability is a risk associated with storing customer data on a web server, as web servers may experience failures or disruptions that can affect the accessibility and usability of customer data, such as hardware faults, network issues, power outages, etc.
However, data availability is not the greatest risk associated with storing customer data on a web server, as it does not affect the security and privacy of customer data. Data integrity is the property that ensures that data are accurate and consistent, and protected from unauthorized modification or corruption. Data integrity is a risk associated with storing customer data on a web server, as web servers may be subject to attacks or errors that can affect the accuracy and consistency of customer data, such as injection attacks, tampering, replication issues, etc. However, data integrity is not the greatest risk associated with storing customer data on a web server, as it does not affect the security and privacy of customer data. Data redundancy is the condition of having duplicate or unnecessary data in a database or system. Data redundancy is not a risk associated with storing customer data on a web server, but rather a result of poor database design or management.

An application's audit trail is a record of all actions or events that occur within or affect an application, such as user activities, system operations, data changes, errors, exceptions, etc. An audit trail can provide evidence and accountability for an application's functionality and performance, and support auditing, monitoring, troubleshooting, and investigation purposes. An IS auditor should ensure that an application's audit trail has adequate security, which means that it is protected from unauthorized access, modification, deletion, or disclosure. Adequate security can help ensure that an audit trail maintains its integrity, reliability, and availability, and prevents tampering or manipulation by attackers or insiders who want to hide their tracks or evidence of their actions. Logs all database records is a possible feature of an application's audit trail, but it is not the most important thing for an IS auditor to ensure, as logging all database records may not be necessary or feasible for some applications, and may generate excessive or irrelevant data that can affect the storage or analysis of the audit trail. Is accessible online is a possible feature of an application's audit trail, but it is not the most important thing for an IS auditor to ensure, as online accessibility may not be required or desirable for some applications, and may introduce security or privacy risks for the audit trail. Does not impact operational efficiency is a desirable outcome of an application's audit trail, but it is not the most important thing for an IS auditor to ensure, as operational efficiency may not be the primary objective or concern of an application's audit trail, and may depend on other factors or trade-offs such as storage capacity, performance speed, or data quality.

The best source of information for an IS auditor to use when determining whether an organization's information security policy is adequate is the risk assessment results. The risk assessment results provide the auditor with an overview of the organization's risk profile, including the identification, analysis, and evaluation of the risks that affect the confidentiality, integrity, and availability of the information assets. The auditor can use the risk assessment results to compare the organization's information security policy with the risk appetite, risk tolerance, and risk treatment strategies of the organization. The auditor can also use the risk assessment results to evaluate if the information security policy is aligned with the organization's objectives, requirements, and regulations.
Some of the web sources that support this answer are:
Performance Measurement Guide for Information Security
ISO 27001 Annex A.5 - Information Security Policies
[CISA Certified Information Systems Auditor - Question0551]