The main difference between Gram positive and Gram negative bacteria is

The correct option is B Cell wall
The Gram-negative cell wall is characterized by presence of outer membrane, a thinner peptidoglycan layer and extensive periplasmic space surrounding the peptidoglycan. The uppermost layer of the outer membrane has lipopolysaccharide while the innermost layer is a typical lipid layer. Presence of membrane channels formed by porin proteins in outer membrane impart make them less penetrable as compared to gram positive cells. Gram positive cells are characterized by presence of single thick layer of cell wall which is made up of peptidoglycan, teichoic acid and lipoteichoic acid. They lack outer membrane and porin and have narrow periplasmic space.

Cell membrane is the unit membrane in both Gram positive and Gram negative bacteria, present beneath cell wall with primarily phospholipids (making up about 30–40% of the membrane mass) and proteins (60–70%).

Both Gram positive and Gram negative bacteria have 70S ribosomes and lack mitochondria (being prokaryotic).

Therefore, the correct answer is option B.