Nostoc is a
(a)Cyanobacteria
(b)Beaded bacterium
(c)Bacteriophage
(d)Parasite

Hint: Nostoc is found in colonies in various environments mainly terrestrial and freshwater. They are autotrophs as they contain photosynthetic pigments to carry out Photosynthesis. Sometimes they are also found in symbiosis with other plants.

Complete answer:
Nostoc is a Cyanobacteria found in colonies in the soil, moist rocks, the bottom of lakes and springs, and sometimes in marine habitats. Sometimes it is seen to have a symbiotic relationship with specific plants such as ancient angiosperm Gunnera and bryophyte hornworts. Nostoc contains photosynthetic pigments, therefore can perform Photosynthesis. Their colonies are composed of filaments of moniliform in a Gelatinous sheath.
In rainy seasons, they form a jelly-like mass and are popularly known as star jelly, witch's butter, witch's jelly, etc.

Additional Information:
Cyanobacteria are prokaryotic organisms that contain free-living photosynthetic bacteria as well as the endosymbiotic plastids. They are also known as blue-green algae and have originated in freshwater or a terrestrial environment.
The bacterium is a prokaryotic microorganism that is found abundantly and in various types ranging from sphere-shaped to rods and spirals. They are found almost everywhere from the soil, water acidic hot springs, radioactive waste, and even in the deep biosphere of the Earth's crust.
Unlike Cyanobacteria or bacterium, the Bacteriophage doesn't have a cell body and hence, comes under the group named virus. These viruses infect a bacteria or archaea cell and replicate there. These are made of certain proteins and encloses a DNA or RNA genome that encodes as few as four genes and as many as hundreds.
A parasite is an organism that lives in a symbiotic relationship with another organism and usually causes harm to it. The organism that carries the parasite is called the host and generally much bigger than the parasite. Parasites can be microparasites and macroparasite depending on their size. Some are obligated i.e. they need the host in order to complete their life cycle and facultative i.e. they do not depend on the host to complete their life cycle. The Parasites that live outside the host are called ectoparasites whereas endoparasites live inside the host organism.

So, the correct answer is “Cyanobacteria”.

Note:
The name nostoc was first introduced by Paracelsus. They belong to the family Nostocaceae of the order Nostocales. There are several species of nostoc found in different regions. Some of them are Nostoc commune, Nostoc azollae, Nostoc flagelliforme, etc. They are consumed as food in Asia due to the fact that they contain proteins and vitamin C. The preferred one in Central Asia is the Nostoc ellipsosporum.