Final answer:
Substance A uses facilitated diffusion, a form of passive transport, which enables the movement of substances from higher to lower concentration through a carrier protein in the cell membrane, without the expenditure of energy.
Step-by-step explanation:
Based on the description provided, substance A is transported via facilitated diffusion. This process is characterized by the movement of substances from an area of higher concentration to an area of lower concentration using a carrier protein in the cell membrane. Facilitated diffusion is a type of passive transport as it does not require cellular energy. This is in contrast to active transport, which moves substances against the concentration gradient and requires energy, typically in the form of ATP. Additionally, simple diffusion is the process by which small, non-polar molecules pass directly through the lipid bilayer without the assistance of a carrier protein.
The specifics mentioned in the question, such as the initial rate of transport changing in different concentrations, suggests that a carrier protein facilitates the transport of substance A. Since it does not move against the concentration gradient and does not require energy, it is not active transport. Hence, the correct transport mechanism is facilitated diffusion.