How can an electrochemical cell be converted into an electrolytic cell?

Strategy: Distinguish between a galvanic cell (converts chemical energy to electrical) and an electrolytic cell (uses electrical energy to drive a non-spontaneous chemical change).

Analysis: A galvanic (electrochemical) cell has a spontaneous reaction producing a potential $E_{\text{cell}}$. To reverse the natural flow and drive the non-spontaneous reaction:

1. Apply an external potential ($E_{\text{ext}}$) in the opposite direction.
2. If $E_{\text{ext}} > E_{\text{cell}}$, the cell is for\ced to run in reverse.
3. In this state, it functions as an electrolytic cell.

$\boxed{\text{Answer: (c) Applying an external opposite potential greater than } E^\circ_{\text{cell}}}$