Cerebrospinal fluid is filtered into the ventricles through this?

The correct answer is choroid plexus.

The choroid plexus is a highly vascularized tissue that projects into the ventricles of the brain. It is responsible for the production of cerebrospinal fluid (CSF), which bathes the brain and spinal cord and provides them with nutrients and oxygen. The CSF is filtered into the ventricles through the choroid plexus, which acts as a barrier to harmful substances.