The expression \( \sqrt{27} \) can be simplified as follows:
\[ \sqrt{27} = \sqrt{9 \times 3} = \sqrt{9} \times \sqrt{3} = 3\sqrt{3} \]
Since \( \sqrt{3} \) is an irrational number, \( 3\sqrt{3} \) is also irrational. The decimal expansion of an irrational number is non-terminating and non-repeating.
Thus, the correct answer is:
D. a non-terminating, non-repeating decimal.