(b) Both, (A) and (R) are true but (R) is not the correct explanation of (A).
The Balkans was explosive due to romantic nationalism, disintegration of the Ottoman Empire, and big power rivalry — not merely Ottoman control.
Both statements are factually correct (the textbook states both), but Ottoman control alone does not explain why the Balkans was the most serious source of nationalist tension. The real cause was the combination of factors: spread of romantic nationalism, Ottoman disintegration, Slavic independence movements, and rivalry among European powers (Russia, Germany, England, Austro-Hungary). So (R) is true but not the correct/complete explanation of (A), making option (b) correct.