according to the map, by 325 c. E ... The northernmost location where christianity had spread was in what region?

The correct answer is - Northwestern Germany.

The Christianity was spreading from place to place, and it was not on a large scale at first, but the places were it became dominant were like little islands on the overall map of Europe. The northernmost place were Christianity spread and became dominant was the northwestern part of nowadays Germany, and it was only a small area where it had spread out in there, while all around it the paganism was still dominant.