Agar-agar is obtained from
A. Chlorella
B. Spirogyra
C. Ulothrix
D. Gelidium

Hint: Agar is a gelatin-like substance derived from seaweed (red algae).

Complete answer:
Agar-agar is a jelly-like gelatin product, made of a mixture of unbranched polysaccharides. Agar is a hydrophilic colloid extracted from seaweeds known as Gelidium. Gelidium is the red algae from class Rhodophyceae and division Rhodophyta. Agar was discovered accidentally in Japan by Minoya Tarozaemon.

Additional Information:
- Agar is used in foods as it is stabilizing, emulsifying, non-digestive, jelly, and resistant to heat.
- Use of agar over gelatin: agar is used in vegan foods as a substitute for gelatin extracted from animal skin and bones and unlike gelatin, agar solidifies at room temperature.
- Agar is insoluble in water but solubilizes in boiling water and that’s the reason why a culture media of bacterial growth, that uses agar as a solidifying agent is heated before setting it for solidification. Once solidified the agar does not melt below 85 degrees celsius.
- The basic monomer of the agar polysaccharide is galactose.
- Agar is isolated from red algae as an amorphous and translucent product sold as bricks, flakes, or in the form of powder.
- In its natural habitat, agar is found in the form of a complex cell-wall constituent of red algae containing the polysaccharide Agarose with calcium and sulfate.

Note: Agar-agar is a plant-based gelatin substance extracted from seaweeds, Red algae. It is used for different purposes like in bacterial culture media, cooking, industrial freezing, in medicines, cosmetics, canning (meat, fish, and poultry), etc.