Introduction:
Shunts are used in PMMC (Permanent Magnet Moving Coil) instruments to measure high currents. They are low-resistance devices that are connected in parallel with the instrument to divert a portion of the current away from the instrument coil. The shunt carries most of the current, while a small fraction of it passes through the instrument coil, producing a proportional deflection on the meter scale. Shunts should possess certain characteristics to ensure accurate and reliable measurements.

Desired Characteristics of Shunts:
1. Stable Resistance: The resistance of the shunt should not vary with time. Any changes in resistance can introduce errors in the measurement. A stable resistance ensures consistent and accurate readings.

2. Material: The shunt must be manufactured using materials that have low temperature coefficients of resistance. Manganin and copper are commonly used because they exhibit minimal resistance changes with temperature.

3. Low Thermal EMF: Shunts should have a low thermal electromotive force (EMF). Thermal EMF is generated when two dissimilar metals are joined and subjected to a temperature gradient. In shunts, this can lead to additional voltage drops and measurement errors. Therefore, shunts should be designed to minimize thermal EMF effects.

4. Temperature Rise: Shunts should be capable of carrying the required current without excessive temperature rise. Excessive temperature rise can cause the shunt to experience thermal expansion, which may alter its resistance and affect the accuracy of measurements. Therefore, shunts should have sufficient thermal conductivity to dissipate heat effectively.

Explanation of the Incorrect Option:
The incorrect option is (b) It must be manufactured using manganin or copper. This statement is incorrect because both manganin and copper are desirable materials for shunts. They possess low temperature coefficients of resistance and are widely used in shunt manufacturing. These materials help maintain stable resistance over a wide range of temperatures, ensuring accurate measurements.

Conclusion:
In summary, the desired characteristics of shunts used in PMMC instruments include stable resistance, materials with low temperature coefficients of resistance (such as manganin or copper), low thermal EMF, and the ability to carry current without excessive temperature rise. The incorrect option stated that shunts must be manufactured using manganin or copper, which is incorrect as these materials are actually desirable for shunt construction.