Lower Order Harmonics and Techniques to Reduce Them
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Lower order harmonics are the harmonics with frequencies that are multiples of the fundamental frequency but have lower multiples. For example, in a system with a fundamental frequency of 50 Hz, the lower order harmonics would include the 2nd harmonic (100 Hz), 3rd harmonic (150 Hz), and so on.

To reduce the lower order harmonics, various techniques can be employed. Some of these techniques include:

1. Phase-shifting transformers: These transformers are used to shift the phase angle of the harmonics, effectively canceling out the lower order harmonics.

2. Active power filters: Active power filters use power electronic devices such as inverters to generate harmonic currents that are out of phase with the harmonics in the system. This cancels out the lower order harmonics.

3. Passive filters: Passive filters consist of passive components such as capacitors, inductors, and resistors arranged in specific configurations to filter out the lower order harmonics.

Higher Order Harmonics and the Use of Filters
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Higher order harmonics are the harmonics with frequencies that are multiples of the fundamental frequency but have higher multiples. For example, in a system with a fundamental frequency of 50 Hz, the higher order harmonics would include the 4th harmonic (200 Hz), 5th harmonic (250 Hz), and so on.

To reduce the higher order harmonics, filters are commonly used. Filters are electronic circuits or devices that attenuate specific frequencies or ranges of frequencies. In the case of higher order harmonics, filters are used to attenuate the harmonics with frequencies that are multiples of the fundamental frequency but have higher multiples.

The Use of Filters to Reduce Higher Order Harmonics
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1. Passive filters: Passive filters can be used to reduce higher order harmonics by providing a low impedance path to the harmonics. These filters are typically designed with specific values of inductors, capacitors, and resistors to create a resonance circuit that attenuates the higher order harmonics.

2. Active filters: Active filters, similar to active power filters mentioned earlier, can also be used to reduce higher order harmonics. These filters use power electronic devices to generate harmonic currents that are out of phase with the harmonics, canceling them out.

3. Digital filters: Digital filters are implemented using digital signal processing techniques. These filters use algorithms to sample, process, and modify the harmonic components of the signal to reduce the higher order harmonics.

Assertion and Reason Analysis
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Assertion (A): The lower order harmonics are reduced by using some technique while the higher order harmonics are reduced by using filters.

Reason (R): The cost of the filter is reduced, and at the same time, the transient response is also improved to a great extent.

Both the assertion and reason are true. The assertion correctly states that lower order harmonics are reduced using techniques, while higher order harmonics are reduced using filters. The reason provides additional information about the benefits of using filters to reduce higher order harmonics, including cost reduction and improved transient response. Therefore, the reason is the correct explanation of the assertion.

Therefore, the correct answer is option A: Both A and R are true, and R is the correct explanation of A.