Correct answer: In a voltaic cell, the electrochemical reaction is spontaneous. In an electrolytic cell, it is nonspontaneous.

Why?:

A voltaic cell uses the energy produced by a chemical reaction to generate electricity. The chemical reaction in the voltaic cell is spontaneous, and produces an electrical current, converting chemical energy to electrical energy.

In contrast the electrolytic cell used in the Hall process consumes electrical energy to drive a chemical reaction. An amount of energy is still needed to drive the reaction process. It is not spontaneous.

Answering this question:

In answering this question, it is important to have knowledge on how voltaic and electrolytic cells work. A voltaic cell is spontaneous because it occurs without the "push" of an outside force. Moreover, it produces energy instead of consuming it. An electrolytic cell on the other hand needs a driving force before it can start. So it is nonspontaneous.

To sum it up, an electrochemical reaction is spontaneous in a voltaic cell while nonspontaneous in an electrochemical cell.