Step 1: Multiply the two expressions using the distributive property (Foil method):
\[
(3 + \sqrt{6})(3 - \sqrt{5}) = 9 - 3\sqrt{5} + 3\sqrt{6} - \sqrt{30}
\]
Step 2: Simplify the expression:
\[
9 - 3\sqrt{5} + 3\sqrt{6} - \sqrt{30}
\]
As none of the radicals can be simplified further or combine to form rational numbers, the expression remains irrational.
Thus, the correct answer is (B) Irrational number.