Answer:
long growing season
Step-by-step explanation:
The characteristic of the South that led to its becoming an agrarian region is a long growing season, which allowed crops to be grown successfully.
The southern region of the United States has a warm and humid climate that allows for a longer growing season compared to other regions. This made it possible for crops such as cotton, tobacco, rice, and sugarcane to be grown successfully. Additionally, the South had a plentiful supply of fertile land, which further encouraged agricultural development. The other options you provided (rocky soil, frequent rain storms, and thick forests) may have posed challenges to agriculture in the South, but the long growing season was the primary factor that enabled its success as an agrarian region.