Answer is (c) 0 Degrees. Because the horizontal component on earth's magnetic field is at its strongest and so the magnetic compass is very stable and accurate. Hence variation at Magnetic Equator is 0 deg.
You're thinking about Dip. Magnetic Variation is the angle between magnetic north and true north.
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yep (d) sounds rite. good explanation given by 'kiitos.patole' on page 37 of this thread.