Explanation:

The frequency of instantaneous power consumed by the geyser is 100 c/s.

To understand this, let's break it down step by step:

1. The geyser is operated from 230 V, 50 c/s mains:
- This means that the geyser is connected to a power supply with a voltage of 230 volts and a frequency of 50 cycles per second (50 Hz).

2. Instantaneous power consumed by the geyser:
- Instantaneous power refers to the power consumed by an electrical device at any given instant in time.
- In an AC circuit, the power consumed is not constant but varies with time due to the alternating nature of the current and voltage.
- The instantaneous power can be calculated using the formula P(t) = V(t) * I(t), where P(t) is the power at time t, V(t) is the voltage at time t, and I(t) is the current at time t.

3. Frequency of the instantaneous power:
- The frequency of the instantaneous power is determined by the frequency of the voltage and current waveforms.
- In this case, the mains supply has a frequency of 50 c/s (50 Hz), which means that the voltage and current waveforms complete 50 cycles in one second.
- Since the power is calculated as the product of voltage and current, the power waveform will also have the same frequency as the voltage and current waveforms.

4. Calculation of the frequency of instantaneous power:
- The frequency of the instantaneous power can be determined by counting the number of cycles in one second.
- In this case, since the mains supply has a frequency of 50 Hz, the power waveform will also have a frequency of 50 Hz.

5. Answer:
- The correct answer is option 'C', which states that the frequency of instantaneous power consumed by the geyser is 100 c/s.