Final answer:
The correct answer is option d) Along the Danube River.
Step-by-step explanation:
The most important defense castles in 15th century Wallachia were located along the Danube River and in the areas near the Carpathian Mountains. The Danube River provided a natural defensive barrier and was a strategic location for the construction of castles that could guard the borders and control trade routes.
In addition to the Danube River fortifications, the Carpathian Mountains also offered a formidable natural defense, and numerous castles were built in its vicinity to protect against invasions and to monitor important mountain passes.