As the electron transport system carriers move electrons by redox reactions, they actively pump across the membrane maintaining a concentration gradient.

Answer;
As the electron transport system carriers move electrons by redox reactions, they actively pump hydrogen ions across the membrane maintaining a concentration gradient. 

Explanation;
Electron transport is a phase of cellular respiration in which majority of ATP is formed. During this phase electron carriers that are reduced during glycolysis and citric acid cycle to NADH+ H+ and FADH2. The carriers then donate electrons and protons to the electron carrier proteins of the ETC, with oxygen being the final acceptor. 

The answer is option a.