Which instruction provides guidance on security classifications and declassification markings?

The appropriate instruction that provides guidance on security classifications and declassification markings is OPNAVINST 5510.1. This instruction outlines the Navy's policies regarding the classification and safeguarding of national security information, detailing the procedures for identifying, classifying, and declassifying sensitive information. It ensures that personnel understand their responsibilities and the importance of protecting classified information properly, which is essential for maintaining national security.

In this context, the other options do not address security classifications and declassification markings. SECNAVINST 5210.11D relates to the management of Navy records and the process for their retention and disposal, but it does not focus explicitly on security classification guidance. OPNAVINST 1420.1 pertains to Navy officer programs and advancement-related policies rather than security protocols. NAVPERS 15627 concerns personnel management and is not involved with security classifications. Thus, OPNAVINST 5510.1 is the definitive source for the specific guidance needed about security classifications and declassification markings.