Sampling rate conversion by rational factor involves converting a signal from one sampling rate to another using a rational factor, which is a ratio of two integers. This process can be achieved through interpolation.

Interpolation:
Interpolation is the process of estimating new data points within a given set of known data points. In the context of sampling rate conversion, interpolation is used to increase the number of samples in a signal when converting from a lower sampling rate to a higher sampling rate. The additional samples are created by estimating the values between the existing samples using various interpolation techniques such as linear, polynomial, or spline interpolation.

Decimation:
Decimation, on the other hand, is the process of reducing the number of samples in a signal. In the context of sampling rate conversion, decimation is used to decrease the number of samples when converting from a higher sampling rate to a lower sampling rate. However, in this scenario, decimation is not performed when converting a signal using a rational factor.

Interpolation or Decimation:
When performing sampling rate conversion by a rational factor, interpolation is always performed, regardless of whether the conversion is from a lower sampling rate to a higher sampling rate or vice versa. The interpolation process is required to generate additional samples or estimate the values between the existing samples.

However, decimation is not performed when converting a signal using a rational factor. Decimation is only performed when the conversion factor is an integer, i.e., when the sampling rate is an integer multiple or divisor of the original sampling rate.

Conclusion:
In summary, when performing sampling rate conversion by a rational factor, interpolation is always performed to increase the number of samples or estimate the values between the existing samples. Decimation, on the other hand, is not performed in this scenario. Therefore, the correct answer is option A: Interpolation.