Final answer:
OCSP provides a real-time protocol for verifying certificates (D), offering an efficient alternative to CRLs for ascertaining a digital certificate's revocation status.
Step-by-step explanation:
The OCSP protocol provides D: a real time protocol for verifying certificates. OCSP stands for Online Certificate Status Protocol, and it is used to obtain the revocation status of an X.509 digital certificate without requiring CRLs (Certificate Revocation Lists). When a device or application needs to verify the validity of a digital certificate, instead of downloading the entire CRL, it can make a real-time query to an OCSP responder. This makes the certificate verification process more efficient, especially in real-world scenarios where the use of certificates is abundant and time-sensitive.
The OCSP protocol is a critical component in maintaining the security of a digital communication channel and is widely used in various secure internet protocols, such as HTTPS.