These bacteria grow anaerobically on cellulosic material , produce large amount of methane along with CO2 and H2, and are collectively called as methanogens . Example(s) of such bacteria are
Methanobacterium
Methanobrevibacter
Methanococcus
all of these
As discussed in Microbes in Production of Biogas -
Biogas -
Biogas is a mixture of gases (containing predominantly methane) produced by the microbial activity and which may be used as fuel.
Hence, the correct option is (d).