Final answer:
Only Gram-negative bacteria have an outer membrane, which contains lipopolysaccharides and lipoproteins, distinguishing them from the simpler cell wall structure of Gram-positive bacteria. Therefore, the most appropriate correct option is B.
Step-by-step explanation:
The student's question pertains to the structural differences between Gram-positive and Gram-negative bacteria, specifically regarding the presence of an outer membrane. Gram-negative bacteria have a more complex cell wall structure, which includes a thin peptidoglycan layer and an additional outer membrane that contains substances such as lipopolysaccharides (LPS) and lipoproteins. In contrast, Gram-positive bacteria have a simpler cell wall structure, which consists of a thick peptidoglycan layer and is anchored to the cell membrane by lipoteichoic acid, but they do not possess an outer membrane. The correct answer is that the presence of an outer membrane is a characteristic of Gram-negative bacteria.