Single-cell protein can be obtained from
A. Bacteria
B. Algae
C. Fungi
D. All of the above

Hint: Single Cell Protein or SCP are also known as microbial proteins. The protein content in microorganisms can be used as a substitute for other protein-rich food sources such as milk and cheese for human consumption. The culture or production of SCPs has many advantages over traditional agricultural methods.

Complete answer:
i. The bacteria employed in the production of single-cell proteins are Cellulomonas and Alcaligenes.
ii. Algae utilized as SCPs are Chlorella which is green algae and Spirulina which is a cyanobacterium or blue-green algae. They are rich in vitamins and have a high protein content.
iii. The fungi involved in the production of SCPs are Saccharomyces, Trichoderma and Rhizopus. Trichoderma uses agricultural wastes as its substrates and can be cultured in a large amount. These species have a high protein content.
Single-cell proteins develop when microbes ferment waste materials (including wood, straw, cannery, and food-processing wastes, residues from alcohol production, hydrocarbons, or human and animal excreta). Thus, microorganisms like bacteria, fungi and algae are the unicellular organisms which are used for the production of Single-Celled Proteins. The advantages of Single Cell Proteins over traditional agricultural methods are that since the sources of these proteins are microbes possessing a very rapid rate of growth. Thus, a large quantity of food can be produced at a very short span of time. These materials have a very high content of protein compared to traditional sources such as vegetable and dairy sources. No wastage occurs in these sources, since the whole organism can be utilized, unlike certain parts of crops, which are supposed to be discarded. These materials can be digested to a larger extent compared to other sources.
Hence, the correct answer is option D.

Note: The term SCP was coined in 1966 by Carroll L. Wilson. Autotrophic SCP represents options of fail-safe mass food-production which can produce food reliably even under harsh climate conditions. The problem with extracting single-cell proteins from the wastes is the dilution and cost.