The correct answer is A) comparing the specific gravity of the electrolyte in each cell. The specific gravity of the electrolyte in each cell of a lead-acid storage battery is the best method for detecting a weak or dead cell. This is because the specific gravity directly corresponds to the state of charge of the cell - a lower specific gravity indicates a weaker or discharged cell. Visually inspecting the electrolyte levels (C) or taking an open circuit voltage test (D) do not provide as reliable an indication of cell health as the specific gravity measurement. Taking each cell's temperature (B) is not the primary method for detecting a weak cell.