Which one is limbless amphibian?

Ichythophis is the limbless amphibian; Ichthyornis is a fossil aquatic bird; Ichthyostega is an extinct amphibian; Ichthyosaurus is an extinct aquatic reptile.

Ichthyophis is a type of limbless amphibian, also known as a "caecilian." These amphibians do not have arms or legs. They are similar to earthworms in appearance, having long, cylindrical bodies that help them move through the soil. Despite being amphibians they spend most of their lives underground or in moist environments, like forests, and are typically found in tropical areas. 

Here’s a quick breakdown of the other options: 

A. Ichthyornis: This is an extinct bird from the Cretaceous period, not an amphibian. 

B. Ichthyostega: This is one of the earliest known amphibians with limbs, so it doesn't match the description of a limbless amphibian. 

D. Ichthyosaurus: This is an extinct marine reptile, not an amphibian. So, Ichthyophis is the correct choice because it is a limbless amphibian species.