The strength of an acid is determined by its ability to donate protons (H⁺ ions) in water. This is measured by its degree of ionization. A strong acid ionizes completely, furnishing a high concentration of H⁺ ions, while a weak acid only partially ionizes. Therefore, the correct factor is the concentration of hydrogen ions furnished by ionization, not the total number of hydrogen atoms, density, or oxygen content.

Final Answer: concentration of hydrogen ions furnished by ionisation.