Identify the strong electrolyte.

Strong electrolytes are substances that completely dissociate into ions in aqueous solutions. They generally include strong acids, strong bases, and soluble salts.

$\text{CaCl}_2$ is a soluble salt that dissociates completely into $\text{Ca}^{2+}$ and $\text{Cl}^{-}$ ions in aqueous solution, making it a strong electrolyte.

$\text{Ca(OH)}_2$ is a strong base but is only sparingly soluble in water.

$\text{NH}_3$ is a weak base and $\text{HCN}$ is a weak acid, meaning they only partially dissociate in water and are therefore weak electrolytes.

Answer: $\text{CaCl}_2$