Step 1: Evaluate each option.
\(\sqrt{25} = 5\) (rational), \(\sqrt{81} = 9\) (rational), \(\frac{5}{3}\) (rational), \(5 + \sqrt{2}\) (irrational).
Step 2: Reasoning.
A sum of a rational number and an irrational number is always irrational.
Step 3: Conclusion.
Hence, \(5 + \sqrt{2}\) is the only irrational number among the options.