A regional bank connects two data centers 80 km apart with sustained 40 Gbps replication, low latency, and full control of encryption keys. It leases dark fiber and needs deterministic performance without sharing bandwidth. Which DWDM-based leased optical connectivity use case best fits these requirements?

DWDM over leased dark fiber is correct because it lights customer-controlled optics on dedicated glass, multiplexing multiple wavelengths onto one fiber pair to deliver sustained 40 Gbps with protocol-transparent, ultra-low-latency photonic transport over 80 km, preserving deterministic performance since no bandwidth is shared and allowing the bank to deploy its own encryptors and retain full custody of keys. The carrier-managed lit wavelength approach fails on tenancy and control concept because the provider lights and manages the transponders and fiber, so even with strong SLAs for capacity and latency the bank does not lease dark fiber nor retain physical-layer autonomy. The carrier-managed encryption variant fails on key custody concept because although it provides correct DWDM dedicated transport, handing encryption to provider-held keys violates the explicit full control requirement critical for regulatory separation and key rotation. The burstable oversubscribed restoration variant fails on performance assurance concept because restoration and oversubscription are designed for efficiency and recovery, not for guaranteed sustained throughput, introducing contention and variable latency unsuitable for synchronous replication. Exam caveat: distinguish leased dark fiber plus customer DWDM from leased lit wavelengths plus managed DWDM, as both mention DWDM and 40 Gbps but imply opposite operational control. Practical check: verify distance budget and dispersion compensation for 80 km, confirm dedicated wavelengths with no oversubscription in the contract, and confirm bank-owned encryption with HSM-backed key management before acceptance.