Atropical depression can become more powerful by: a. joining with a tropical disturbance. b. absorbing energy from the stratosphere. c. slowing down their rotation rate. d. gaining energy from warm water. the answer is d. no one answered so here it is. : )

D. By gaining energy from warm water
   As warm air from warm water rises in a tropical depression, the air pressure below the depression becomes lower. As higher air pressure air from areas outside the depression move into the same space, that warm air from warm water becomes wetter, warmer and rises as well. The result is a stronger system of clouds and wind powered by the heat of the ocean which causes the depression to become more and more powerful.