, statement (A) is correct because methanogens are a type of archaebacteria that inhabit marshy areas and are responsible for methane production . Statement (B) is correct as Nostoc is identified as a filamentous cyanobacterium (blue-green algae) that fixes atmospheric nitrogen in specialised cells . Statement (D) is correct because Mycoplasma are unique organisms that completely lack a cell wall and possess the ability to survive without oxygen . Statement (C) is incorrect because chemosynthetic autotrophic bacteria obtain energy by oxidising various inorganic substances such as nitrates and ammonia to produce ATP, rather than synthesising cellulose from glucose . Therefore, the correct combination is (A), (B), and (D) .