Amplitude modulation (AM) is a technique used to combine a carrier signal with a modulating signal in order to transmit information. In the process of producing AM, one method is to add the carrier and modulating signal and then pass it through a square law device followed by a low pass filter.

Explanation:

Square Law Device:
A square law device is a nonlinear device that produces an output signal proportional to the square of the input signal. In the context of AM, the square law device is used to multiply the carrier signal with the modulating signal.

Low Pass Filter:
A low pass filter is an electronic circuit that allows low-frequency signals to pass through while attenuating high-frequency signals. In the context of AM, the low pass filter is used to remove the high-frequency components generated by the square law device, leaving behind the modulated signal.

Working:
1. The carrier signal, which typically has a higher frequency, and the modulating signal, which carries the information to be transmitted, are added together.
2. The resulting signal is then passed through the square law device.
- The square law device multiplies the carrier and modulating signal, resulting in the generation of sum and difference frequencies.
- The sum frequencies are unwanted and need to be filtered out.
- The square law device ensures that the amplitude of the carrier signal is varied according to the modulating signal.
3. The output of the square law device is then passed through a low pass filter.
- The low pass filter attenuates the sum frequencies and allows the desired modulated signal to pass through.
- The low pass filter removes the high-frequency components generated by the square law device, resulting in the extraction of the modulated signal.
4. The filtered output signal represents the amplitude-modulated signal, which can be further amplified and transmitted through a medium.

Advantages of using a square law device followed by a low pass filter:
- The square law device ensures that the amplitude of the carrier signal is varied according to the modulating signal, resulting in a faithful reproduction of the modulating signal.
- The low pass filter removes unwanted high-frequency components, resulting in a cleaner and more efficient transmission of the modulated signal.
- This method of AM production is relatively simple and cost-effective.

Hence, the correct answer is option 'A': a square law device followed by a low pass filter.