Best Solution for User Demand

D.

The security architect is looking for a system that meets the following objectives: reduced transaction time, increased security and message integrity, and improved cryptographic security. The system must also be used in an environment with a broad user base where many asynchronous transactions occur every minute and must be publicly verifiable.

Option A suggests an internal key infrastructure that allows users to digitally sign transaction logs. This solution may provide increased cryptographic security and message integrity, but it does not address the need for reduced transaction time and public verifiability.

Option B suggests an agreement with an entropy-as-a-service provider to increase the amount of randomness in generated keys. While this solution can increase cryptographic security, it does not address the need for reduced transaction time, message integrity, and public verifiability.

Option C suggests a publicly verified hashing algorithm that allows revalidation of message integrity at a future date. This solution can provide message integrity and public verifiability, but it does not address the need for reduced transaction time and improved cryptographic security.

Option D suggests an open distributed transaction ledger that requires proof of work to append entries. This solution can provide reduced transaction time, improved cryptographic security, and public verifiability. Proof of work ensures that only legitimate entries are appended to the ledger, which increases security. Additionally, the distributed nature of the ledger ensures that it is publicly verifiable, which meets the requirements of the architect.

Therefore, the BEST solution that meets all of the architect's objectives is option D, an open distributed transaction ledger that requires proof of work to append entries.