Explanation:

- Option 1: NaCl (Sodium chloride)

- Common table salt. It does not directly help the thyroid make thyroxine, but iodized salt is used to prevent iodine deficiency.

- Option 2: KCl (Potassium chloride)

- Used as a salt substitute. It has no role in the production of thyroxine.

- Option 3: Cholesterol

- This is important in making steroid hormones, not thyroid hormones.

- Option 4: Iodine

- Essential for thyroid gland to synthesize the hormone thyroxine (T4). The thyroid collects iodine from the bloodstream and uses it to make thyroxine.