The correct answer to this open question is the following.
Although the question does not include a map, we can say that most densely populated areas of Europe were central and western Europe, what today is Germany, Poland, Lithuania, The Czech Republic, and Slovakia.
The Black Death arrived in Europe from Asia in 1348 and spread throughout Europe. Historians believe that Black Death killed almost 60% of the population in that region. Some believe that trade was the reason the epidemy arrived in Europe from Asia. At that time, there were many trade routes by sea and land, and many rodents carried the Black Death.