The subarachnoid space ends at:

**Core Concept**
The subarachnoid space is a potential space between the arachnoid mater and the pia mater, two of the meningeal layers surrounding the brain and spinal cord. This space contains cerebrospinal fluid (CSF) and allows for the absorption of CSF.

**Why the Correct Answer is Right**
The correct answer is at the level of the foramen magnum, where the spinal cord terminates and becomes the conus medullaris. The subarachnoid space extends from the base of the brain down to the level of the conus medullaris, where it ends. The arachnoid mater and pia mater merge and fuse at the level of the conus medullaris, marking the end of the subarachnoid space.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a correct anatomical landmark.
**Option B:** This option is incorrect because it is not a recognized anatomical landmark marking the end of the subarachnoid space.
**Option C:** This option is incorrect because it is not a correct anatomical landmark marking the end of the subarachnoid space.

**Clinical Pearl / High-Yield Fact**
The subarachnoid space ends at the level of the conus medullaris, which is an important landmark for lumbar punctures. A lumbar puncture should be performed below the level of the conus medullaris to avoid damaging the spinal cord.

**Correct Answer:** C.