Question

ion is needed by the thyroid gland to make the hormone thyroxine.
a iodide
b chloride
c fluoride

Explanation:

Thyroxine is a thyroid hormone. The thyroid gland actively takes up iodide ions ($I^-$) from the blood. These iodide ions are then used in the synthesis of thyroxine. Chloride ($Cl^-$) and fluoride ($F^-$) ions do not play a role in thyroxine synthesis.

Answer:

A. Iodide