The correct option is B Juvenile hormone
Corpora allata is an endocrine gland which generates juvenile hormone. It actually plays a crucial role in metamorphosis. As long as this hormone is present in the blood, the molting epidermal cells lay down a larval cuticle. In the last larval stage, the juvenile hormone is no longer produced and the insect undergoes metamorphosis into an adult.
So, the correct answer is option D.