Sampling rate conversion by the rational factor I/D is accomplished by cascade connection of interpolator and decimator.

Interpolation:
- Interpolation is the process of increasing the sampling rate of a signal.
- It involves inserting additional samples between the existing samples of a signal to create a higher sample rate version of the signal.
- The additional samples are usually generated by using an interpolation filter.

Decimation:
- Decimation is the process of decreasing the sampling rate of a signal.
- It involves reducing the number of samples in a signal while maintaining its essential characteristics.
- Decimation is typically achieved by using a decimation filter.

Rational Factor:
- The rational factor I/D represents the ratio of the desired output sampling rate to the input sampling rate.
- The numerator I represents the interpolation factor, which denotes how many additional samples are inserted between the original samples.
- The denominator D represents the decimation factor, which denotes how many samples are discarded from the original samples.

Cascade Connection:
- In order to perform sampling rate conversion by the rational factor I/D, an interpolator and a decimator are connected in cascade.
- The input signal is first passed through the interpolator, which increases its sampling rate by a factor of I.
- The interpolated signal is then passed through the decimator, which reduces the sampling rate by a factor of D.
- The output signal of the decimator is the final signal with the desired sampling rate.

Advantages of Cascade Connection:
- The cascade connection of interpolator and decimator allows for flexible and efficient sampling rate conversion.
- It provides the ability to perform both interpolation and decimation simultaneously.
- The interpolation filter and decimation filter can be designed independently, optimizing their respective operations.
- It allows for the adjustment of the interpolation and decimation factors independently, providing more flexibility in sampling rate conversion.

Therefore, the correct answer is option 'B' - Cascade.