Answer: B. Subsistence crops were mostly grown by poor families in the South.
Explanation: Subsistence means to support yourself at a mininum level. This essentially means to earn as much or grow as much as you need, and nothing more for extra things, such as jewelry or cake. This indicates a poor family grew subsistence crops, so slaves wouldn't have been involved. This removed A and D as the answer.
Many of the Southern colonies were rich in this time, so there weren't many poor people to grow subsistence crops. This removed C.