Step 1: Stomatal movement depends on rapid, reversible ion flux across guard cell membranes, not structural nutrients.
Step 2: N, S, and P mainly support protein, amino acid, and energy metabolism, they do not drive fast osmotic changes.
Step 3: \(K^+\) ions are pumped in and out of guard cells, changing water potential and turgor pressure.
Step 4: This makes Potassium the regulating element.
\[ \boxed{\text{Option (2): K}} \]