When you multiply \( \frac{3}{5} \) (a fraction) and \( 3\pi \) (where \( \pi \) is an irrational number), the product will be:
\[ \frac{3}{5} \times 3\pi = \frac{9\pi}{5} \]
Since \( \frac{9\pi}{5} \) includes \( \pi \), which is irrational, the product remains irrational.
Thus, the correct response is:
an irrational number.