Difference in Gram +ve and Gram -ve bacteria is due to

The correct option is A Cell wall
The cell wall of certain bacteria shows a characteristic reaction to the stain devised by C. Gram. Those bacteria which retain the stain are known as Gram positive and those which do not retain the stain are called Gram negative.

• The cell wall in bacteria contains peptidoglycan, a polymer of N-acetyl glucosamine, N-acetyl muramic acid and amino acids.
• Gram-positive cell walls contain a thick layer of peptidoglycan that encircles the cell.
• Gram-negative cell walls contain a thin layer of peptidoglycan between the cytoplasmic membrane and the outer membrane.