Step 1: Rank cations by crustal abundance and exchange affinity: \( \text{Ca}^{2+} > \text{Mg}^{2+} > \text{K}^{+} > \text{Na}^{+} \).
Step 2: Calcium's divalent charge lets it bind exchange sites more tightly than sodium or potassium.
Step 3: Aluminium dominates only in special, strongly acidic soils, not generally.
Step 4: The correct option is Calcium.