Correct option: (a) administered antithyroid substance like thiourea
Explanation:
Thyroxine, the principal hormone of thyroid gland, is required for tissue differentation and metamorphosis (the rapid transformation from the larval to the ***** form) in amphibians. So in its absence of presence of antithyroid substance e.g. thiourea, tadpoles or axolotl remain in the larval stage indefinitely. Thiourea is an organic compound of carbon, nitrogen, sulphur and hydrogen, with the formula CSN2H4 or (NH2 ) 2CS. It is similar to urea, except that oxygen atom is replaced by a sulphur atom.
Antithyroid agents and the thiouracils and mercaptoglyoxalines owe their activity to the presence of the potentially free thiol group, which by its read reduction of elementary iodine to iodide ion,
2R-+12-+ R--*-R + 2I
interferes with the iodination of tyrosine in the thyroid protein and hence inhibits the synthesis of the thyroid hormone.
