Looking at the answer options:

'To check their position only NDBs should be used to obtain bearings and these should then be converted to true bearings using the magnetic variation at the station' --> INCORRECT. "Only NDBs" and why not VORs?

'Their DR position should be cross-checked with a bearing from a VOR, which should then be converted to true using the magnetic variation at the aircraft' --> INCORRECT. In Northern Canada, VORs may be oriented with reference to true north, rather than magnetic north and so not need conversion.

'The Earth's vertical field strength is reduced and the ability of the compass to accurately sense Magnetic North is reduced' --> INCORRECT. The vertical component of the vertical field becomes stronger nearer the pole, it is the horizontal component that weakens.

'The magnetic compass may be unreliable or unusable; basic inertial navigation requires NO special procedures' --> CORRECT. The vertical component of the vertical field becomes stronger nearer the pole, reducing the accuracy and reliability of the compass. Use of inertial navigation however, is not affected by it's location over the Earth. The ability of the system to determine speed and position is not affected by proximity to the magnetic pole.