Introduction:
An operational amplifier (op-amp) is a versatile electronic device that amplifies the difference between two input voltages. The gain of an op-amp is the ratio of the output voltage to the input voltage. The gain of an op-amp can vary depending on the frequency of the input signal. In this question, we are asked to determine at which frequency the gain of an op-amp will be maximum.

Explanation:
To understand why the gain of an op-amp is maximum at 1 Hz, let's consider a few key factors:

1. Open-loop gain:
The open-loop gain of an op-amp is the gain without any external feedback. In an ideal op-amp, the open-loop gain is infinite. However, in practical op-amps, the open-loop gain is very high but finite. The open-loop gain typically decreases with increasing frequency.

2. Unity gain frequency:
The unity gain frequency, also known as the gain-bandwidth product (GBP), is the frequency at which the open-loop gain of an op-amp drops to 1. At frequencies below the unity gain frequency, the open-loop gain is high. Above this frequency, the gain decreases.

3. Frequency response:
The frequency response of an op-amp is a graph that shows the variation of gain with frequency. It is typically represented by a Bode plot, which consists of a magnitude plot (gain) and a phase plot. The frequency response of an op-amp depends on its internal compensation and external components.

4. Gain in the frequency domain:
In the frequency domain, the gain of an op-amp can be expressed as a function of frequency. At low frequencies, the gain is high, but as the frequency increases, the gain decreases. This behavior is due to the internal compensation of the op-amp.

Why the gain is maximum at 1 Hz:
Based on the above factors, we can conclude that the gain of an op-amp is maximum at frequencies below the unity gain frequency. The unity gain frequency is the frequency at which the gain drops to 1. Since the unity gain frequency is typically higher than 1 Hz, the gain of an op-amp will be maximum at frequencies below 1 Hz.

Conclusion:
In conclusion, the gain of an operational amplifier is maximum at frequencies below the unity gain frequency. In this question, the correct answer is option 'A' because 1 Hz is the lowest frequency among the given options. At frequencies above the unity gain frequency, the gain of an op-amp decreases. It is important to consider the frequency response and open-loop gain characteristics of an op-amp when designing circuits to ensure optimal performance.