The correct answer is C) over an electrically heated platinum filament. Combustible gas indicators operate by drawing an air sample into the instrument and passing it over an electrically heated platinum filament. The presence of combustible gases in the air sample alters the electrical resistance of the platinum filament, which is then measured and displayed as a gas concentration reading. This principle of operation, using the heated platinum filament, is the key feature that makes C the correct answer. The other options are incorrect because they do not accurately describe the core operating mechanism of a combustible gas indicator. Mixing the air sample with nitrogen (A), measuring its specific gravity (B), or igniting it with a sparking device (D) are not the primary methods used by these types of instruments.