Final answer:
Sodium contributes to approximately 90% of the extracellular fluid's osmotic activity.
Step-by-step explanation:
The extracellular fluid consists of various components, but the major cation that contributes to around 90% of the osmotic activity is sodium. Sodium is responsible for maintaining osmotic pressure and plays a crucial role in regulating the balance of water and electrolytes across cell membranes. It is present in relatively low concentrations compared to other electrolytes and helps balance the osmotic pressure between the intracellular and extracellular environments.