Explanation: The power factor in an AC circuit is defined as the cosine of the phase angle ($\theta$) between the voltage and the current. It represents the ratio of true power (or real power) to apparent power. Given a phase angle of 45 degrees between the voltage and the current in an R-L circuit, we calculate the power factor as: Power Factor = cos($\theta$) Power Factor = cos(45°) The value of cos(45°) is approximately 0.707. Therefore, an R-L circuit with a 45-degree phase angle has a power factor of 0.707. * 0.866 would correspond to a phase angle of 30 degrees (cos 30°). * 1.0 would indicate a phase angle of 0 degrees, meaning a purely resistive circuit with no reactive component. * 0.5 would correspond to a phase angle of 60 degrees (cos 60°).