The excretory system is absent in the Platyhelminthes.
 

Concept: In Platyhelminthes, there is the flame cell which helps in the excretion of the waste materials.
In Platyhelminthes, there is only one opening, which is the mouth through which egestion and ingestion.
So in Platyhelminthes, the anus is not present but that does not mean that Platyhelminthes excretory system is absent.
Hence, option A is the correct answer.