Final answer:
The filled-in blank for Volta's arrangement should be 'None of the above', as the piece of zinc and copper would be used to construct a voltaic cell due to their different reactivities, where zinc is more reactive than copper.
Step-by-step explanation:
When Volta placed a piece of paper soaked in salt water between pieces of different metals, he was essentially constructing a simple galvanic cell. A galvanic cell requires two metals of differing reactivities to function. According to the options provided, Volta placed a piece of paper soaked in salt water in between a piece of zinc and a piece of copper. The correct answer is None of the above, as it suggests that the missing metal is neither zinc, nor silver, nor both metals equally reactive. The activity series, which lists metals in order of their reactivity with water and other substances, shows that zinc is more reactive than copper, and this difference in reactivity is essential for the operation of a voltaic cell. Therefore, in historical context, Volta would likely have used copper as the other metal in his experiments, rather than a metal of equivalent reactivity or one that is non-reactive with water, such as silver.