Answer:
Step-by-step explanation:
The coil is rotating about a vertical axis so the plane of the coil must be horizontal . The radius of the coil will cut the vertical component of the earth magnetic field . Due to it a potential difference will be induced between the center of the coil and its circumference . This potential difference will be proportional to speed of rotation of the coil.
The face of coil is lying in horizontal plane so its rotation will not create any change in the flux passing through it as total flux passing through it will remain zero all the time.
Hence it is the vertical component of earth's magnetic field which is important to calculate induced flux in the coil.