Final answer:
The method not suitable for a hydrometer is measuring the amount of heavy metals in a water bottle as hydrometers measure specific density of liquids, not solids.
Step-by-step explanation:
The method that is not applicable for obtaining an accurate reading from a hydrometer is option C) Measuring the amount of heavy metals in a water bottle. A hydrometer is designed to measure the specific density or relative density of liquids and is not suitable for detecting or measuring solid substances like heavy metals in solutions. This could be done using other analytical methods.
Options A), B), and D) can be associated with the use of a hydrometer. Hydrometers can measure the amount of water in substances like milk (A) or alcohol (B) because they are buoyant liquids. In the case of D), a hydrometer is specifically designed to check the charge level of a lead-acid accumulator by measuring the specific gravity of the battery acid. This is indicative of the state of charge of the battery.