In active transport, carrier proteins are used, which use energy in the form of ATP, to transport molecules against concentration gradient of cell membrane, cell wall, or along concentration gradient of cell wall, cell membrane.

In active transport, carrier proteins which are located at membrane, use energy in the form of ATP to transport molecules against concentration gradient of cell membrane. Transportation rate reaches maximum when all carrier proteins are being used or are saturated. These proteins are very specific to certain substances and are sensitive to inhibitors. So, the correct answer is option C.