To simplify the expression \( 5^9 \cdot 5^{-13} \), we apply the property of exponents that states \( a^m \cdot a^n = a^{m+n} \).
Thus, we have:
\[ 5^9 \cdot 5^{-13} = 5^{9 + (-13)} = 5^{-4} \]
Now, to express this with only positive exponents, we can use the property \( a^{-n} = \frac{1}{a^n} \):
\[ 5^{-4} = \frac{1}{5^4} \]
So, the equivalent expression with only positive exponents for \( 5^9 \cdot 5^{-13} \) is:
\[ \frac{1}{5^4} \]
Among the provided options, the correct response is:
1/5^4.