Final answer:
Reactance is necessary for creating magnetic fields in motors and transformers, helps maintain voltage stability, and limits reactive power flow. While a unity power factor is often desired, it is not always possible due to varying types of loads. Reactance can therefore be adjusted to improve power factor in electrical systems.
Step-by-step explanation:
Reactance in a system is required for several reasons, and even though a unity power factor is often ideal, it is not always possible or desirable.
Reactive power is necessary for the operation of motors and transformers because it helps to create the magnetic fields required for their functioning (option B). Reactance, both inductive and capacitive, plays a crucial role in this process. Reactance can also help limit the flow of reactive power and maintain voltage stability (option D), essential for the consistent operation of electrical grids. In some cases, having a non-unity power factor can be a result of the type of load and may be unavoidable (option C).
For instance, if the voltage leads the current, as is the case with an inductive load, a capacitor can be placed in series to compensate reactance and bring the power factor closer to unity. Conversely, if the voltage lags the current, an inductor may be used. Moreover, at resonance, inductive reactance equals capacitive reactance, and power factor correction can be precisely tailored to achieve power factor improvements.