Iodine is necessary for the synthesis of the hormone thyroxine.
Explanation: Iodine is a crucial component of thyroid hormones, specifically thyroxine (T4) and triiodothyronine (T3), which are produced by the thyroid gland.
Why other options are incorrect:
(a) Adrenaline: Adrenaline is produced by the adrenal medulla and does not require iodine for its synthesis.
(c) Auxin: Auxin is a plant growth hormone and is not related to iodine metabolism.
(d) Insulin: Insulin is produced by the pancreas and does not require iodine for its synthesis.