Cell wall is only absent in the following group of bacteria:

Explanation

The correct option is D: Mycoplasmas. These bacteria are distinctive in lacking a cell wall, which makes them naturally resistant to antibiotics such as penicillin that target cell wall synthesis. Mycoplasmas rely on a specialized cell membrane containing sterols for stability and are often pathogenic in animals. In contrast, Staphylococci, Pseudomonas, and Diplococcus pneumoniae all possess cell walls made largely of peptidoglycan, which is essential for their structural integrity and vulnerability to certain antibiotics.