What Is the Meaning of Carrier Proteins and Pumps Becoming

What is the meaning of carrier proteins and pumps becoming saturated?

A) These transporters have reached a maximal net flux, beyond which further increases in concentration no longer affect their rate.
B) These transporters have become entirely depleted and can no longer transport without further ATP production.
C) They have a hydrogen molecule at every possible carbon bond and are, therefore, a solid at room temperature.
D) These transporters have reached the point where only an increase in the transported molecule will change the net flux.
E) Saturation kinetics (demonstrated graphically as a line curving upward as a function of concentration) indicates that transport will increase as concentrations increase.