Which one of the following  bacteria is  cell  wall  deficient?: (A) Escherichia  coli, (B) Salmonella  typhi, (C) Mycoplasma, (D) Treponema pallidum
Answer is C. Mycoplasma: 

This is a naturally occurring bacteria without cell walls. 
Mycoplasma is classiﬁed as an independent bacterial genus. 
They don't require a hypertonic environment for maintenance and are stable in culture medium.