What substance is described as functioning to allow: neuromuscular excitability, secretory activity, membrane permeability, & maintaining correct osmotic relationship between cells & ECF?

The correct answer is "sodium chloride".

Explanation:

Even though the excess of sodium chloride (common table salt) have the detrimental effect of causing high blood pressure, sodium chloride in the correct amounts are highly beneficial for the body. Sodium chloride allows for neuromuscular excitability, secretory activity, membrane permeability, & maintaining correct osmotic relationship between cells & ECF. The chloride anion that comes from salt is a major contributor to the osmotic pressure gradient between the ICF and ECF, which ultimately leads to cells and ECF communication.