(A) Both (A) and (R) are correct and (R) is the correct explanation of (A).
The Zollverein customs union was formed in 1834 at Prussia's initiative to create a unified economic territory allowing unhindered movement of goods, people and capital.
Source: Chapter 1, Section on Nationalism in Europe
Both statements are factually correct as per the textbook. The Reason directly explains why the Zollverein was formed — to enable free economic movement — making it a correct explanation of the Assertion. Students often confuse option (A) and (B); choose (A) only when R logically explains A, which it does here.