In five-kingdom classification system, the kingdom that includes the blue-green algae nitrogen-fixing bacteria and methanogenic archaebacteria is
A. Plantae
B. Fungi
C. Protista
D. Monera

Hint: Living organisms are classified into five different kingdoms. This is decided based on factors such as nutrition mode, reproductive mode, phylogenetic relationship and structural body organization. This classification was given by RH Whittaker.

Complete answer:
> Option A: Plantae: - As the name itself suggests, this kingdom includes all the plants. They cannot move and are fixed to a substrate. Plants perform photosynthesis because they have structures like chloroplast and chlorophyll pigments. Their life cycle has alternating haploid gametophyte and diploid sporophyte generation.

> Option B: Fungi: - Microorganisms such as mushrooms and yeast are primarily included in kingdom Fungi. Fungi are very fond of moist and warm places and hence it can best grow in such places. They have thread like structures called hyphae that combine together forming mycelium.

> Option C: Protista: - They consist of all eukaryotes but still are a very different group of organisms. They majorly include unicellular amoeba, some multicellular seaweeds and algae. Many protists are dangerous and can cause diseases like malaria and sleeping sickness.

> Option D: Monera: - This kingdom includes nitrogen-fixing bacteria, methanogenic archaebacteria along with blue-green algae. Archaebacteria are a bit different because they have a bit of difference in their cell wall structure.

Hence, the correct answer is option (D).

Note: Apart from the four kingdoms mentioned above, there is a kingdom Animalia that is also included. The organisms included in this kingdom lack a cell wall and photosynthetic pigments. They are heterotrophic and need to feed on others. The examples of this kingdom are Annelids, Mollusca, Porifera, Cnidaria and some others. Classifying organisms into different groups makes it more easy and convenient to study each and every aspect of them in detail.