Final answer:
In purple bacteria, two electrons are required to move two hydrogen ions (H+) across the cell membrane.
Step-by-step explanation:
The question is asking about the movement of electrons across the cell membrane in purple bacteria, specifically during the process of cellular respiration through the electron transport chain (ETC). In the context of the electron transport in purple bacteria, for every two hydrogen ions (H+) that are moved across the membrane, two electrons are required to facilitate this transport. The correct answer to this question is therefore: b) 2.