To simplify the expression \( 13^{-5} \cdot 13^{-11} \) using the properties of exponents, we can apply the rule that states when multiplying like bases, we add the exponents:
\[ 13^{-5} \cdot 13^{-11} = 13^{-5 + (-11)} = 13^{-16} \]
Now we want to express this with only positive exponents. The rule for converting a negative exponent to a positive exponent is:
\[ a^{-n} = \frac{1}{a^n} \]
So, we have:
\[ 13^{-16} = \frac{1}{13^{16}} \]
Thus, the equivalent expression with only positive exponents is:
\[ \frac{1}{13^{16}} \]
Looking at the given options, the correct equivalent expression is:
1/13^16.