The correct answer is: B. Perpendicular
The two fixed coils in a Weston frequency meter are perpendicular to each other. This is because the magnetic field produced by one coil will not affect the other coil if they are perpendicular. This allows the meter to measure the frequency of an alternating current without being affected by the current’s amplitude or phase.
If the two coils were parallel, the magnetic field produced by one coil would affect the other coil. This would cause the meter to read a frequency that is different from the actual frequency of the alternating current.
If the two coils were inclined at 60° or 15°, the magnetic field produced by one coil would still affect the other coil, but to a lesser extent than if they were parallel. This would cause the meter to read a frequency that is closer to the actual frequency of the alternating current, but it would still be slightly inaccurate.
Therefore, the two fixed coils in a Weston frequency meter must be perpendicular to each other in order to achieve the most accurate measurement of the frequency of an alternating current.