## **Core Concept**
Regressive metamorphosis refers to a type of metamorphosis where the larva is more complex and advanced than the adult form. This process involves the degeneration of the larval structures to form a simpler adult. It is commonly observed in certain parasitic animals.

## **Why the Correct Answer is Right**
The correct answer, **D. Dracunculus**, shows regressive metamorphosis. Dracunculus, also known as the guinea worm, is a parasitic nematode. Its lifecycle involves water fleas (copepods) as intermediate hosts. When infected copepods are ingested, the larvae are released and develop into adult worms in the human host. The larval stages within the copepods are more developed than the adult worms, which is a characteristic of regressive metamorphosis.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option does not specify an organism; hence, we cannot assess its correctness directly. However, based on common knowledge, many organisms undergo different types of metamorphosis.
- **Option B:** Without a specific organism mentioned, it's challenging to directly refute this option. Typically, organisms like frogs undergo progressive metamorphosis, not regressive.
- **Option C:** Similarly, without details, we can't directly assess this option. However, many marine animals exhibit various forms of metamorphosis.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that regressive metamorphosis is often seen in parasitic organisms where the larval form is more complex than the adult. This is a critical concept in parasitology and can be a distinguishing feature for certain types of parasites.

## **Correct Answer: D. Dracunculus**