The function \( f(x) = \frac{x^2 + 2x - 15}{x^3 + 1} \) has a vertical asymptote at \( x = -1 \) and a horizontal asymptote at \( y = 0 \). Therefore, the answer is:
\[
\boxed{\text{D: vertical asymptote: } x = -1; \text{ horizontal asymptote: } y = 0}
\]