The correct option is B peristomium
The mouth is present in the first segment of the earthworm called peristomium. The mouth of earthworm is covered by a fleshy lobe called prostomium. It is sensory in nature and serves as a wedge to force open cracks in the soil as the earthworm burrows. During embryonic development, protostomes are the animals that undergo protostomy, that is, the blastopore develops into the mouth and the opposite end develops into the anus. Annelids are an example of this kind of development.