Which of these power system elements is known to consume reactive power?

The correct choice highlights that heavily loaded transmission lines and motor loads are known to consume reactive power. In power systems, reactive power is essential for maintaining voltage levels necessary for the transfer of real power. When motors, especially induction motors, operate, they typically require reactive power to establish and maintain the magnetic fields necessary for operation.

In addition, heavily loaded transmission lines can also lead to significant reactive power consumption, as they may exhibit increased impedance leading to voltage drops. The flows of reactive power in a power system help in managing and balancing the voltage levels, hence the presence of reactive power consumption in these components is crucial for their efficient operation.

The other choices consist of elements that either produce or manage reactive power rather than consume it. Shunt reactors, for instance, are used in mitigating reactive power under certain conditions by absorbing it, while synchronous condensers can either generate or absorb reactive power depending on their operating conditions. Generators and capacitors are typically considered sources of reactive power as they provide it to the system to help support voltage levels. Lightly loaded transmission lines and lighting loads generally consume less reactive power or may even behave differently regarding their real and reactive power consumption.