Agar-agar commonly used in bacterial cultures and medication is obtained from
A. Sargassum
B. Gelidium
C. Ulothrix
D. Ulva

Hint: Agar-agar is the jelly-like substance, which is obtained from the algae, and the jelly material is nothing but the polysaccharide branches of the cell wall, and this agar-agar is used for a wide number of varieties which include deserts.

Complete answer:
Agar-agar is a jelly-like substance obtained from the Red algae.
This agar mainly contains two types of materials, which are linear polysaccharides called agarose and small molecules of the heterogeneous mixture of the agaropectin.
These are nothing but the structural supporting substances present in some species of algae, where these substances are released while boiling.
Agar-agar is used in a wide range of varieties, it is used as an ingredient in the desert in so many Asian places.
And it is also used as a substrate for culture media of microorganisms and it is used for culturing. Whereas that agar agar for bacterial culture usually obtains from ulothrix.
It is also used as a laxative, and it is also used as an appetite suppressant.
The gelling agent of the agar-agar is obtained from the polysaccharides of the cell wall of the red algae.
Tengusa and ogonori are some species of red algae which are used for commercial purposes.
In chemical terminology, agar-agar is nothing but the poly subunits of the sugar galactose.
So the correct option is C.

Note: Agar-agar contain, 80% of the dietary fiber, so it can be used as an intestinal regulator, and in some countries like Japan, this agar-agar in water, it swells by absorbing water and makes the stomach fuller, and this diet has got appreciation in the USA in obesity studies.