**Core Concept**
The formation of cerebrospinal fluid (CSF) involves a complex process that occurs in the choroid plexus of the ventricles in the brain. This process is essential for maintaining the correct balance of fluids within the cranial cavity and for cushioning the brain. The choroid plexus is composed of specialized epithelial cells that produce CSF.

**Why the Correct Answer is Right**
The correct answer is related to the process of active transport, where ions and water are moved across the cell membrane of the choroid plexus epithelial cells. The process involves the Na+/K+-ATPase pump, which maintains the ionic balance necessary for CSF production. The choroid plexus epithelial cells also possess aquaporin-1, a water channel protein that facilitates the movement of water into the CSF. This active transport process results in the production of CSF, which is then circulated throughout the ventricles and eventually absorbed into the bloodstream.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not accurately describe the process of CSF formation. While it is true that the choroid plexus is involved in CSF production, this option does not provide a specific mechanism or step involved in the process.
**Option B:** This option is incorrect as it is unrelated to the formation of CSF. The process of diffusion is not directly involved in CSF production, although diffusion may play a role in the movement of ions and water within the choroid plexus.
**Option C:** This option is incorrect as it is not a specific step involved in CSF formation. The term "passive transport" is too general and does not accurately describe the active transport process that occurs in the choroid plexus.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that CSF is produced at a rate of approximately 500 mL/day and is cleared from the cranial cavity through the arachnoid villi, which are small projections of the arachnoid mater into the venous sinuses. This balance between production and clearance is essential for maintaining the correct balance of fluids within the cranial cavity.

**Correct Answer: C.