Which of the following lacks a well-defined nucleus?

Consider the following four statements $(A-D)$ and select the option which includes all the correct ones only.
$(A)$ Archaebacteria differ from other bacteria in having a different cell wall structure.
$(B)$ Bacteria as a group show the most extensive metabolic diversity.
$(C)$ All Eubacteria have chlorophyll ' $a$' pigment similar to green plants.
$(D)$ Mycoplasma can survive without oxygen.

The structure that participates in bacterial sexual reproduction is called .........

The cell wall-less prokaryote among the following is:

There is no alternation of generation in $Escherichia$ $coli$ because of the absence of

In which of the following organisms is the cell wall absent in their cells?