In which kingdom are prokaryotes included?

What is the importance of Cyanobacteria?

$Thermococcus$,$Methanococcus$ and $Methanobacterium$ exemplify:

$A$ true nucleus is absent in

Cyanobacteria and heterotrophic bacteria have been clubbed together in eubacteria of kingdom-Monera as per the 'five kingdom classification' even though the two are vastly different from each other. Is this grouping of the two types of taxa in the same kingdom justified? If so,why? Discuss the cellular composition of cyanobacteria and heterotrophic bacteria that make them introduced in eubacteria.

Which of the following statements is correct?