Iodine test can detect
A. Carbohydrates
B. Nucleic acids
C. Lipids
D. Proteins

Hint: It is a test to detect the presence of starch, where the sample changes into blue-black color after addition of a few drops of potassium iodide solution in the sample.

Complete Answer:
- The Iodine Test for Starch is used to indicate the presence of starch in biological materials. The reaction in iodine test is due to the formation of polyiodide chains from the reaction of starch and iodine.
- The amylose in starch forms helices where iodine molecules assemble, forming a dark blue or black color. When starch is hydrolyzed into tiny carbohydrate units, the blue-black color is not developed. Therefore, this test can also indicate completion of hydrolysis when a color change does not occur.
- The iodine test is used to check the availability of starch like carbohydrates in a sample. Starch changes to a deep "blue-black" color upon addition of aqueous solutions of the triiodide anion, because of the production of an intermolecular charge transfer component (complex).

Thus, the correct option is A i.e. Carbohydrates.

Note: In iodine-starch test, the triiodide anion quickly produces a deep blue-black color upon contact with starch. The deepness of the color reduces with rising the temperature and with the presence of water soluble organic solvents such as ethanol. The test cannot be done at very low pH, because starch is hydrolyzed in this condition.