Grey powder (chalk + mercury) is used in ?

Ans. is 'c' i.e., Dactylography A latent finger print may be developed by dusting the area with coloured powders to provide a contrast, and its pattern is recorded by photography. It can also be examined by oblique lighting. The commonly used powder is 'grey' powder (chalk and mercury), but white powders (lead carbonate or French chalk) are used for dusting dark surfaces.