A 36-year-old healthy Punjabi man working in an abattoir presented with pustule on hand which turned into ulcer. Staining with which of the following will best help in diagnosis?

Pustule on the hand of an abattoir worker suggests the diagnosis of cutaneous anthrax. Anthrax bacillus is gram positive and non acid fast. When blood films containing anthrax bacilli are stained with polychrome methylene blue for a few seconds and examined under microscope an amorphous purplish material is noted around the bacilli. This represents the capsular material and is characteristic of the anthrax bacillus. This is called M'Fadyen's reaction and is employed for presumptive diagnosis of anthrax in animals. When it is stained with Sudan black B, fat globules may be made out within the bacilli.