Final answer:
The density of the liquid in which a hydrometer with a density of 0.750 g/ml is submerged 91% of its volume is 0.6825 g/ml.
Step-by-step explanation:
When a hydrometer with a density of 0.750 g/ml floats in a liquid with 91% of its volume submerged, the density of the liquid can be calculated using the principle of buoyancy, according to Archimedes' Principle. The hydrometer's density is equal to the density of the liquid when the submerged volume percentage matches. Given that 91% of the hydrometer is submerged, the density of the liquid is thus 0.91 times the density of the hydrometer since the hydrometer is made to float at the level that matches its density to the density of the liquid.
To calculate the density of the liquid: 0.750 g/ml (density of hydrometer) × 0.91 (submerged volume percent) = 0.6825 g/ml. Therefore, the correct answer is 0.6825 g/ml, which corresponds to option a.