Which engine designation system is applicable to all armed services?

The correct choice pertains to the Military Standard system, which provides specifications and guidelines for various military designations. MIL-STD-1812 specifically outlines the format for the identification of military engines across all branches of the armed services. This standardization is crucial for interoperability and logistical purposes, ensuring that each service branch can easily understand and utilize equipment from other branches.

In contrast, the other options do not encompass all armed services. While NSN (National Stock Number) is indeed used across military branches for item identification, it does not strictly apply to engine designation. MIL-STD-1622 is focused on the general specification for military aircraft, not a comprehensive engine designation system. NAVFAC-900 is related specifically to the Navy's facility-related specifications and does not serve the purpose of engine designation across all services. Thus, MIL-STD-1812 is the appropriate answer as it establishes a unified system applicable to all armed services for identifying military engines.