Final answer:
The cerebrospinal fluid (CSF) has a higher concentration of calcium ions (Ca2+) compared to the blood.
Step-by-step explanation:
The correct answer is C. Ca2+.
In the cerebrospinal fluid (CSF), calcium ions (Ca2+) have a higher concentration than in the blood. Calcium plays a crucial role in various physiological processes, including nerve function and muscle contraction.
While potassium (K+), hydrogen ions (H+), bicarbonate ions (HCO3-), and sodium ions (Na+) are also important in the body's fluid balance, they do not have a higher concentration in CSF compared to the blood.