Question

the electromotive force or cell potential for a voltaic cell is zero. can be positive or negative depending on cell conditions. is always negative. is always positive.

Explanation:

A voltaic (galvanic) cell is a type of electrochemical cell that generates electrical energy from spontaneous redox reactions. The cell potential (electromotive force) of a voltaic cell is a measure of the driving force for the spontaneous reaction. By definition, spontaneous redox reactions in voltaic cells have a positive cell potential because the Gibbs free energy change ($\Delta G$) for the reaction is negative (since $\Delta G = -nFE_{cell}$, and for a spontaneous reaction $\Delta G < 0$, so $E_{cell}$ must be positive when $n$ (moles of electrons) and $F$ (Faraday's constant) are positive values). A cell potential of zero would imply equilibrium (no net reaction), negative cell potential would indicate a non - spontaneous reaction (which is characteristic of an electrolytic cell, not a voltaic cell), and the cell potential of a voltaic cell is not dependent on conditions in the sense of being able to be negative (as that would change it to an electrolytic cell operation). So the cell potential for a voltaic cell is always positive.

Answer:

D. is always positive.