Final answer:
The characteristic of active transport is that it includes movement against a concentration gradient, uses ATP or a proton gradient as energy, and has a saturable uptake rate; thus, all of the choices are correct. The correct option is d.
Step-by-step explanation:
Among the options provided, the characteristic of active transport is that it can move materials against a concentration gradient, requires the use of ATP or a proton gradient as a source of metabolic energy, and has a saturable uptake rate. Therefore, all of the choices are correct.
Active transport is a biological process in which cells expend energy, usually in the form of adenosine triphosphate (ATP), to move substances across a cell membrane. This process can transport materials from a region of lower concentration to one of higher concentration, thereby working against both the concentration and electrical gradients.
This movement is typically mediated by specific membrane proteins that function like pumps, and their activity is often regulated, which results in a saturable rate of transport.
Therefore, the correct option is d.