Which of the following is a requirement for a law enforcement officer to administer a field sobriety test?

A law enforcement officer is required to observe unsafe driving behavior as a basis for administering a field sobriety test. This observation provides the necessary justification for suspecting that a driver may be under the influence of alcohol or drugs. The purpose of field sobriety tests is to evaluate a driver's physical and cognitive abilities, which can be impaired by substances. When an officer witnesses erratic driving, such as swerving, speeding, or braking suddenly, it raises suspicions that warrant further investigation through these testing methods.

The other options, while they may relate to different aspects of traffic enforcement, do not serve as foundational requirements for conducting field sobriety tests. Roadside checkpoints, for instance, are established for general monitoring and prevention but do not require a specific incident of unsafe driving to justify a sobriety test. Similarly, a driver’s age or the passenger capacity of the vehicle are not indicators of impairment and do not require assessment through field sobriety tests. Therefore, the observation of unsafe driving behavior is a critical element that legitimizes the need for further assessment of a driver's sobriety.