Which database is used to determine if a subject has previously been in the U.S.?

The correct choice used to determine if a subject has previously been in the U.S. is the EID/EARM system. This database is specifically designed to track immigration-related information, including the arrival and departure records of individuals. It allows border patrol agents and other law enforcement officials to access important information regarding a person's immigration history, which is essential in assessing their current status.

In contrast, other databases serve different purposes. For example, the NCIC (National Crime Information Center) primarily maintains criminal justice information and is used for tracking criminal records. TECS (Treasury Enforcement Communications System) offers data related to customs and border protection but does not focus specifically on immigration history. The NGI (Next Generation Identification) system is designed to enhance biometric identification services, primarily for law enforcement purposes, and doesn't directly monitor immigration status or previous U.S. entries.