The inability to create a Roll-Up Summary field in this context can be attributed to:

B. The organization has Advanced Currency Management enabled. When Advanced Currency Management (ACM) is enabled, it affects how currency data is managed and aggregated, impacting the functionality of roll-up summary fields that involve currency amounts across different records.

To address this limitation, consider using custom reporting or a custom solution like a trigger or a scheduled batch class to manually calculate and update the total.

For more information on limitations with ACM, consult Salesforce's help article on Advanced Currency Management.