Which can readily respire without oxygen?
A)Anabaena
B)Saccharomyces
C)Mushroom
D)Fish

Hint: Respiration is defined as the process in which carbon dioxide is released out and oxygen is carried inside that is absorbed. Respiration is the essential process for breakdown of food from complex material to the simple ones. It helps to keep the organs of the body in a well-positioned manner.

Complete answer:
Based on the presence or absence of oxygen two types of respiration are present.
Aerobic respiration:- the respiration that occurs in the presence of oxygen and results in the formation of water , and energy.
Anaerobic respiration:- the respiration that occurs in the absence of oxygen and results in the formation of ethanol or sometimes lactic acid.

Now let us find solution from given options:
Anabaena : It is a kind of eubacteria. It requires oxygen for the respiration process. It fixes nitrogen in the roots of leguminous plants.
Saccharomyces : It is a kind of yeast and yeast undergoes fermentation which do not require the presence of oxygen and respiration occurs without oxygen. It leads to formation of lactic acid.
Mushroom : Mushroom is a kind of fungi and needs oxygen for its respiration so it undergoes aerobic respiration only.
Fish : Fishes are the group of member vertebrates. In these structures the notochord is replaced by the vertebral column. Fishes are aquatic animals so they have to breathe more and more faster to obtain oxygen as water has less oxygen when compared to the terrestrial environment.

Hence, the correct answer is option (B)

Note: Respiration is a multistep, enzyme mediated biochemical process of oxidative breakdown of organic compounds inside living cells releasing small packets of energy at various steps. It is, therefore, a catabolic and exergonic process. The organic compounds that undergo oxidative breakdown in respiration are called respiratory substrates. The most common respiratory substrate is glucose.