Final answer:
Rising ADP levels in the mitochondria signal the cell to increase electron flow through the electron transport chain in order to produce more ATP, hence the correct answer is C) Electron flow through the ETC increases.
Step-by-step explanation:
The question pertains to the effect of rising ADP levels in the mitochondria on the rate of electron flow through the electron transport chain (ETC). In response to increased ADP levels, which indicate that a cell is consuming more ATP, the rate of electron transport through the ETC will increase. This occurs because the cell needs to produce more ATP to meet its energy demands. The ratio of ADP to ATP is a regulatory signal: when ADP levels increase relative to ATP, it signals the cell to accelerate the ETC and, consequently, increase the production of ATP through oxidative phosphorylation.