Agar-agar is obtained from
A) Green algae
B) Red algae
C) Brown algae
D) yellow green algae

Hint:Agar-agar is obtained from the cell wall of a red algae. Algae is the primitive of the first organism of the plant Kingdom. They are a thallus like structure that lacks vascular tissues and there is very less differentiation of true tissues in them.

Complete answer:
The best Agar is obtained from the cell wall of red algae "Gelidium". The other sources for agar include Gracilaria and Gigartina. Both of them are also the varieties of red algae.
Now let us understand all the four options.
Option A is green algae. Green algae is green due to the presence of chlorophyll a and b. Common examples of Green algae are cladophora, chlamydomonas, etc.
Option B is red algae. Red algae is red in colour due to the presence of chlorophyll a and d along with phycoerythrin. Common examples as we know are Gracilaria, Gelidium, etc. These are only used to obtain the best Agar from cell walls. This is the correct answer.
Option C is brown algae. Brown algae is brown in colour due to the presence of chlorophyll a and c along with fucoxanthin. Examples of brown algae are fucus, laminaria, etc.
Option D is yellow green algae. These are commonly known as cyanobacteria now and are placed under Monera Kingdom of the five kingdoms of classification.

Hence , the correct answer is option B.

Note:Bacteria are harmful as well as useful. They perform many useful activities like decaying of organic matter, improving soil fertility, preparation of alcohol, production of acetic acid and many more. They are harmful as they cause food poisoning, water pollution and several kinds of diseases. In humans, these bacteria cause diseases like influenza, tetanus, pneumonia, plague, syphilis, strep throat and many more.