The auditor's greatest concern when reviewing data inputs from spreadsheets into the core finance system would be undocumented code that formats data and transmits directly to the database. This is because undocumented code can introduce errors, inconsistencies, and security risks in the data processing and reporting. Undocumented code can also make it difficult to verify the accuracy, completeness, and validity of the data inputs and outputs, as well as to trace the source and destination of the data. Undocumented code can also violate the principles of segregation of duties, as the same person who creates the code may also have access to the data and the database.
The other options are not as concerning as undocumented code, although they may also pose some risks. A lack of complete inventory of spreadsheets and inconsistent file naming may make it challenging to identify and locate the relevant spreadsheets, but they do not directly affect the quality or integrity of the data inputs.
The department data protection policy not being reviewed or updated for two years may indicate a lack of awareness or compliance with the current data protection regulations, but it does not necessarily imply that the data inputs are compromised or inaccurate. Spreadsheets being accessible by all members of the finance department may increase the risk of unauthorized or accidental changes to the data, but it can be mitigated by implementing access controls, password protection, and audit trails.
References:
ISACA, CISA Review Manual, 27th Edition, 2019, p. 2261
Five Common Spreadsheet Risks and Ways to Control Them2
GREATEST Concerns When Reviewing Data Inputs from Spreadsheets3

Here's the breakdown, considering
the need for DLP visibility in the cloud:
Verified Answer
C: Employ a cloud access security broker (CASB).
Very Short Explanation
CASBs are specifically designed to enhance visibility and control over cloud-based data. They can monitor data flows, enforce security policies, and often have DLP capabilities built-in, making them the ideal solution in this scenario.
References
ISACA Resources (Glossary): Definitions of Cloud Access Security Broker (CASB) highlight their role in cloud security and governance.
Industry Research (Gartner, etc.): Research on CASB tools emphasizes their ability to address visibility and control challenges for cloud data.