need a helping hand here...

[D driver]

To measure the distance from 70N 20W to 70N 150E on a polar stereographic, use the latitude scale around approximately:
A.
70°N.
B.
80°N.
C.
85°N.
D.
90°N.

ans-B 80 N

[Mach E Avelli]

I have not looked at a polar stereo graphic chart in about 40 years, but would guess that if you draw a line from the first position to the destination, that line would pass very nearly through the Pole. You can guess this from the change in longitude being nearly 180 degrees and from the form of the chart which is not unlike a spoked wheel, with the pole being the hub, the spokes the longitudes and the rim one line of latitude with all the others concentric circles to the rim.
Latitude 80 north is equidistant between 70 north and 90 north, hence the most accurate for obtaining the distance scale.

[Intruder]

Process of elimination. Without looking at a chart:

90N? YGBSM!
70N? Not on a great circle!

That leaves 80 and 85N. Since the max latitude is 90N, the average latitude is likely closer to 80 than 85.