Iodine stains starch in plant cells blue-black

Steps:

• Iodine solution tests for starch by forming a blue-black complex with it.
• Plant leaf cells store starch as energy reserve, often in chloroplasts or amyloplasts.
• Animal skin cells lack starch, containing glycogen instead, which does not react with iodine.
• Compare diagrams: identify starch-containing structure in plant cell versus animal cell parts.

Why B is correct:

• B labels the starch granule in the plant cell, which reacts with iodine per the chemical test for polysaccharides.

Why the others are wrong:

• A is likely the nucleus, present in both cells but stains brown with iodine, not blue-black.
• C is probably cytoplasm, which does not contain starch and shows no specific iodine reaction.
• D may be the cell wall or vacuole, neither of which stains blue-black with iodine.

Final answer: B