Concept: A leaf chosen for a starch test must be free of starch at the start, so that any blue-black colour later is due to newly formed starch.
Explanation:Plants kept in darkness cannot photosynthesise, so they gradually lose the starch already stored in their leaves.
By early morning, before sunrise, the leaf contains little or no starch.
If the leaf is picked before sunrise, the test result becomes reliable because no starch was present initially.
If the leaf is picked after exposure to sunlight, starch may already be present from earlier photosynthesis.
Similarly, after sunset, the leaf still contains starch made during the day.
Thus, only such a leaf will show a true result when tested with iodine solution, which turns blue-black only in the presence of starch.
Answer: C. before sunrise.