Throughout Europe (both West and East) and extending into western Russia there is a physical region that is known for being the most productive agricultural lands in the area. What is the name of that physical region?

The North European Plain.

Explanation:

A physical region that is known for being the most productive agricultural lands in Europe is the North European Plain which exists mostly in Denmark, Poland, Belgium, Germany, the Netherlands, with little parts of northern France and the Czech republic. As the main use of the Northern European Plain is commercial farming, with few natural vegetation remaining, while the region also covers heath, bogs, and lakes.