Introduction:
Efficiency in the transmission of bulk AC power is an important consideration in electrical systems. The power factor of the load plays a significant role in determining the efficiency of power transmission. In this case, the maximum efficiency is achieved when the power factor of the load is less than unity (less than 1) and leading.

Explanation:
1. Power Factor:
- Power factor is the ratio of real power (active power) to apparent power in an AC circuit. It represents the efficiency of power utilization.
- Power factor can be lagging (inductive load) or leading (capacitive load), or unity (resistive load).
- Lagging power factor occurs when the load has inductive components like motors and transformers, which cause the current to lag behind the voltage waveform.
- Leading power factor occurs when the load has capacitive components like capacitors, which cause the current to lead the voltage waveform.
- Unity power factor occurs when the load is purely resistive, with no reactive components. The current is in phase with the voltage waveform.

2. Power Transmission Efficiency:
- The efficiency of power transmission is defined as the ratio of real power delivered to the load to the total power transmitted.
- Real power is the power that is actually consumed by the load and performs useful work.
- Apparent power is the product of voltage and current in an AC circuit and represents the total power transmitted, including both useful and reactive power.

3. Maximum Efficiency:
- The maximum efficiency in power transmission is achieved when the power factor of the load is less than unity and leading.
- Leading power factor reduces the reactive power component in the transmission line, which reduces the losses due to reactive power flow.
- Lagging power factor increases the reactive power component, resulting in additional losses in the transmission line.
- Unity power factor results in the transmission of only real power, but it does not provide any power factor correction or reduction in losses.

Conclusion:
In summary, the maximum efficiency in the transmission of bulk AC power is achieved when the power factor of the load is less than unity and leading. This reduces the losses due to reactive power flow and improves the overall efficiency of the power transmission system.