The correct option is C Thyroid
Metamorphosis is a biological process during which the larva develops into adult. The larva of frog is called tadpole which is very distinct from the adult frog. The larva undergoes metamorphosis to become into an adult frog. The thyroid gland produces thyroxine, which is an essential hormone in the metamorphosis of frog.
Thymus and adrenal glands are endocrine glands that play a major role in regulating physiological activities.
Pancreas is a mixed gland which produces both hormones and enzymes.