The testable hypothesis is that if acid is added to a solution, then the pH of the solution will go down, which can be scientifically tested.
Hypothesis: The testable hypothesis among the options provided is: If acid is added to a solution, then the pH of the solution will go down. This hypothesis follows the standard format of 'If... then...' and is specific and measurable, as changes in pH can be quantitatively observed and tested.
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