The correct answer is option 'C': Increase in spacing between the phase conductors.

Explanation:
Inductance is a property of an electrical circuit that opposes changes in current. It is caused by the magnetic field generated around a current-carrying conductor. The inductance of a power transmission line depends on several factors, including the diameter of the conductor, line length, spacing between the phase conductors, and the load current carried by the conductor.

- Increase in diameter of the conductor:
The inductance of a conductor is inversely proportional to its diameter. A larger diameter conductor has a lower inductance, as the magnetic field produced by the current is spread over a larger area. Therefore, an increase in the diameter of the conductor would result in a decrease in inductance, not an increase.

- Decrease in line length:
The inductance of a transmission line is directly proportional to its length. A longer transmission line has a higher inductance because the magnetic field generated by the current has a longer path to travel. Therefore, a decrease in line length would result in a decrease in inductance, not an increase.

- Increase in spacing between the phase conductors:
The inductance of a power transmission line is significantly affected by the spacing between the phase conductors. When the spacing between the conductors is increased, the magnetic field produced by the current is spread over a larger area. This results in a decrease in the magnetic flux linkage between the conductors and a decrease in inductance. Therefore, an increase in spacing between the phase conductors leads to a decrease in inductance.

- Increase in load current carried by the conductor:
The inductance of a transmission line is not directly affected by the load current carried by the conductor. The load current determines the amount of current flowing through the conductor, but it does not directly affect the inductance of the transmission line.

In conclusion, the correct answer is option 'C': Increase in spacing between the phase conductors. Increasing the spacing between the phase conductors decreases the inductance of a power transmission line.