Gentian violet colouration of gram-positive bacteria is due to -
Peptidoglycan
Capsule
Cell membrane
None of the above
GRAM STAINING 
The gram stain differentiates bacteria into two broad groups
1. Gram-positive → Resist decolourization and retain primary stain, appear violet.
2. Gram-negative → Decolourised by organic solvents, and therefore, take the counterstain, appearing red. Smear stained with gentian violet (primary stain)
↓ Application of dilute solution of iodine
↓ Decolourisation with an organic solvent, e.g., ethanol
↓ Counterstaining by carbol fuchsin
↓ ↓ ↓ Retain primary stain (violet)
↓ Gram-positive Take colour of counterstain (red)
↓ Gram-negative 
Why do gram-positive organisms retain primary stain, but not gram-negative organisms.

 In gram-positive bacteria, there is a thick layer of peptidoglycan (cell wall) just outside the bacterial cytoplasmic membrane.
 This thick wall traps the primary stain (crystal violet) which is not decolorized by ethanol.
 In gram-negative bacteria this peptidoglycan (cell wall) layer is thin and just outside this peptidoglycan layer is outer cell membrane (which is not present in gram-positive bacteria).
 This outer cell membrane is partially dissolved by ethanol, thus washing out the crystal violet and allowing the counterstain to take