Which bar indicates that an explorer has completed one year of service?

The Tenure Bar signifies that an explorer has completed one year of service within the program. It is specifically designed to recognize the duration of participation and commitment of the explorer. Achieving this bar reflects an explorer's dedication and stability in the program, which may be important for future advancement opportunities.

The other options serve different purposes. The Perfect Attendance Bar acknowledges consistent attendance, the Emergency Preparedness Bar is awarded for skills and knowledge related to emergency situations, and the Law Enforcement Service Bar typically recognizes completion of specific law enforcement-related training or activities. Each of these serves to recognize different achievements, but the Tenure Bar specifically relates to the length of service, making it the correct choice for this question.