The most likely driver for approving additional network security budget is that the audit revealed that the existing architecture contained security controls that could be easily bypassed. This indicates fundamental weaknesses in defense-in-depth and suggests that attackers could gain access to sensitive systems or data despite the presence of controls.

Option A (unused budgets) is not a strategic reason for approving security investment. Option B (compliance reports requested by customers) may influence investment in compliance initiatives, but it does not explain the need for an in-depth defense architecture. Option D (PCI DSS low score) is a compliance-specific issue but would not, on its own, drive a broad architectural budget approval unless PCI was the only focus.

Security audits often uncover systemic flaws—such as flat networks, insufficient segmentation, or single points of failure—that create the conditions for bypassing controls. Addressing these issues requires rearchitecting the environment, introducing layered defenses, and strengthening monitoring capabilities, all of which demand significant budget. Thus, option C aligns with the decision to invest in robust defense-in-depth strategies.