RC oscillators are a type of oscillator circuit that generate stable periodic waveforms using resistor-capacitor (RC) combinations. These oscillators are commonly used in various electronic systems for generating clock signals, timing signals, and frequency references.

RC oscillators can be designed using different configurations, depending on the desired frequency range, stability, and complexity. The number of RC combinations used in an RC oscillator depends on the specific design and requirements of the oscillator.

Here are the options provided in the question and their explanations:

a) One RC combination:
Some simple RC oscillator circuits, such as the RC phase shift oscillator, use only one RC combination to generate the oscillations. In a phase shift oscillator, the RC network provides the required phase shift for positive feedback, resulting in sustained oscillations. However, this type of oscillator may not provide high stability and frequency accuracy.

b) Two RC combinations:
Some more complex RC oscillator circuits, such as the Wien bridge oscillator, require two RC combinations to generate the oscillations. The Wien bridge oscillator uses two RC networks in a bridge configuration to create the necessary phase shift and positive feedback. This type of oscillator can provide better stability and frequency accuracy compared to a single RC combination oscillator.

c) At least three RC combinations:
The correct answer to the question is option 'C', which states that an RC oscillator uses at least three RC combinations. This means that some RC oscillators may require more than two RC combinations to achieve the desired oscillation frequency and stability. These oscillators typically involve multiple stages of RC networks combined with amplifiers or active elements to provide the necessary feedback and frequency control.

d) Either (a) or (b):
This option is incorrect because it does not consider the possibility of RC oscillators that require more than two RC combinations.

In conclusion, an RC oscillator can use either one or two RC combinations, but there are also designs that require at least three RC combinations for achieving the desired oscillation frequency and stability. The specific number of RC combinations depends on the oscillator circuit's complexity and the desired performance characteristics.