Step 1: Understanding Gram staining.
In Gram staining, a differential staining technique, gram-positive bacteria retain the crystal violet stain, appearing purple under the microscope due to their thick peptidoglycan cell wall.
Step 2: Analyzing the options.
(1) Fast green: Incorrect, fast green is not used in Gram staining.
(2) Haematoxylin: Incorrect, haematoxylin is used for staining cell nuclei, not for Gram staining.
(3) Crystal violet: Correct — Crystal violet is the primary stain used in Gram staining to color the cell walls of Gram-positive bacteria.
(4) Safranin: Incorrect, safranin is used as a counterstain for Gram-negative bacteria in the Gram staining procedure.
Step 3: Conclusion.
The correct stain for Gram-positive bacteria is (3) Crystal violet.