The correct answer is option 'D', which means that all of the above modifications are necessary to determine the resistance of a solution using the usual method of measurements on the Wheatstone bridge principle.

Let's discuss each modification in detail and why it is necessary:

1. A.C. should be used:
When measuring the resistance of a solution, it is important to use alternating current (A.C.) instead of direct current (D.C.). This is because A.C. reduces the polarization effects at the electrodes, which can affect the accuracy of the measurement. By using A.C., the resistance measurement becomes more stable and reliable.

2. A conductivity cell is used:
A conductivity cell is a device specifically designed to measure the conductivity or resistance of a solution. It consists of two electrodes, usually made of platinum, that are immersed in the solution. The conductivity cell allows for precise and accurate measurements of the solution's resistance as it provides a fixed path for the current to pass through the solution. Without a conductivity cell, it would be difficult to obtain accurate resistance measurements.

3. Galvanometer is replaced by magic eye or headphone:
In the usual method of measurements on the Wheatstone bridge principle, a galvanometer is used to detect the null point where the bridge is balanced. However, a galvanometer may not provide a clear and precise indication of the null point. Therefore, it is recommended to replace the galvanometer with a magic eye or headphone.

A magic eye is a visual indicator that displays a bright spot when the bridge is balanced, making it easier to determine the null point. A headphone, on the other hand, produces a clear audible sound when the bridge is balanced, providing an alternative method of detecting the null point. By replacing the galvanometer with a more sensitive indicator, the accuracy and precision of the resistance measurement are improved.

By implementing all of these modifications (using A.C., a conductivity cell, and replacing the galvanometer), the usual method of measurements on the Wheatstone bridge principle becomes more reliable and accurate in determining the resistance of a solution.