What method is used to determine the incombustible content after rock dust application?

The correct answer describes the method of collecting and analyzing rock dust with a volumeter to determine its incombustible content. This process involves measuring the volume of the dust after it has been distributed in the mine and assessing its characteristics to ensure compliance with safety regulations.

Using a volumeter allows for a precise measurement of the dust's density, which is crucial in determining how effective the rock dust application is in enhancing safety by minimizing the potential for coal dust explosions. This method is particularly important in underground mining where the accumulation of combustible materials can pose serious risks.

Other methods mentioned, such as spectrometer analysis or standard chemical testing, may assess different qualities of materials but do not specifically focus on volumetric analysis related to incombustible content after rock dust application. Visual inspection, while useful for preliminary assessments, lacks the scientific rigor needed for precise quantification, making volumetric analysis the more reliable choice for this measurement.