Explanation:

Introduction:
Thyroxine and iodine play crucial roles in the development and metamorphosis of amphibians, including tadpoles. Thyroxine is a hormone produced by the thyroid gland, while iodine is an essential element required for the synthesis of thyroid hormones. The presence of trace amounts of thyroxine or iodine in water can have significant effects on tadpole development.

Effect on Tadpole Metamorphosis:
The correct answer to the given question is option B, which states that the addition of a trace of thyroxine or iodine in water containing tadpoles will hasten their metamorphosis. This means that the tadpoles will undergo the process of metamorphosis and transform into adult frogs at a faster rate.

Explanation:
The addition of thyroxine or iodine in water mimics the natural process that occurs in the tadpole's body during metamorphosis. During normal development, the thyroid gland of tadpoles produces thyroxine hormone, which stimulates various cellular and tissue changes leading to metamorphosis.

By adding a trace of thyroxine or iodine in water, the concentration of these substances increases in the tadpole's environment. This higher concentration then activates the tadpole's thyroid gland, leading to an increased production of thyroxine hormone.

The increased thyroxine hormone levels initiate and accelerate the process of metamorphosis in the tadpoles. This process involves the development of limbs, absorption of the tail, and other morphological and physiological changes necessary for the tadpole to transform into an adult frog.

Conclusion:
In conclusion, the addition of a trace of thyroxine or iodine in water containing tadpoles will hasten their metamorphosis. This occurs because the increased concentration of thyroxine or iodine stimulates the tadpole's thyroid gland, leading to an increased production of thyroxine hormone. This hormone then triggers and accelerates the process of metamorphosis, allowing the tadpoles to transform into adult frogs at a faster rate.