A scientist could best determine which ion (F-, Cl-, Br-, or I-) produced the greatest electrical potential in the Experiment by comparing the electrical potential that develops between ice and: a solution of each sodium salt at the same concentration.
a single sodium salt solution at different concentrations.
a single sodium salt solution at different temperatures.
droplets of sodium salt solutions at different collision rates.

a solution of each sodium salt at the same concentration

Explanation:

If a scientist wishes to study the electrical potential that develops between ice  and the sodium salts solution of the ions (F-, Cl-, Br-, or I-) in order to determine which ion produced the greatest electrical potential; he must compare the electrical potential that develops between ice and a solution of each sodium salt at the same concentration.

This means that conclusions can not be reached unless the sodium salt of each ion at the same concentration is compared so as to ensure consistence in the results of the study.

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