The spin-only magnetic moment is calculated using the formula:
\[
\mu_{\text{eff}} = \sqrt{n(n+2)} \, \text{BM}
\]
For \( [Cr(CN)_6]^{3-} \) (d$^3$):
\[
\mu_1 = \sqrt{3(3+2)} = \sqrt{15} \, \text{BM}
\]
For \( [Cr(H_2O)_6]^{3+} \) (d$^3$):
\[
\mu_2 = \sqrt{3(3+2)} = \sqrt{15} \, \text{BM}
\]
Since both have the same electronic configuration, the ratio is:
\[
\frac{\mu_1}{\mu_2} = \frac{\sqrt{15}}{\sqrt{15}} = 1
\]
Thus, the ratio of magnetic moments is 1. The correct answer is (1).