Which of these has the smallest maxima on positive real numbers?

$\textbf{(A)}\ \frac{x^2}{1+x^{12}} \qquad\textbf{(B)}\ \frac{x^3}{1+x^{11}} \qquad\textbf{(C)}\ \frac{x^4}{1+x^{10}} \qquad\textbf{(D)}\ \frac{x^5}{1+x^{9}} \qquad\textbf{(E)}\ \frac{x^6}{1+x^{8}}$
To determine which function has the smallest maximum on the positive real numbers, we need to analyze the maxima of each function given in the options:

\[
\textbf{(A)}\ a(x) = \frac{x^2}{1+x^{12}}
\]
\[
\textbf{(B)}\ b(x) = \frac{x^3}{1+x^{11}}
\]
\[
\textbf{(C)}\ c(x) = \frac{x^4}{1+x^{10}}
\]
\[
\textbf{(D)}\ d(x) = \frac{x^5}{1+x^9}
\]
\[
\textbf{(E)}\ e(x) = \frac{x^6}{1+x^8}
\]

We will find the critical points of each function by setting the derivative equal to zero and solving for \(x\). 

1. **Finding the derivative of \(d(x)\):**

\[
d(x) = \frac{x^5}{1+x^9}
\]

Using the quotient rule, \(\left(\frac{u}{v}\right)' = \frac{u'v - uv'}{v^2}\), where \(u = x^5\) and \(v = 1 + x^9\):

\[
u' = 5x^4, \quad v' = 9x^8
\]

\[
d'(x) = \frac{(5x^4)(1 + x^9) - (x^5)(9x^8)}{(1 + x^9)^2}
\]

Simplify the numerator:

\[
d'(x) = \frac{5x^4 + 5x^{13} - 9x^{13}}{(1 + x^9)^2} = \frac{5x^4 - 4x^{13}}{(1 + x^9)^2}
\]

Set the derivative equal to zero to find the critical points:

\[
5x^4 - 4x^{13} = 0
\]

Factor out \(x^4\):

\[
x^4(5 - 4x^9) = 0
\]

This gives us two solutions:

\[
x^4 = 0 \quad \text{or} \quad 5 - 4x^9 = 0
\]

Since \(x > 0\), \(x^4 = 0\) is not valid. Solve for \(x\):

\[
5 = 4x^9
\]

\[
x^9 = \frac{5}{4}
\]

\[
x = \left(\frac{5}{4}\right)^{\frac{1}{9}}
\]

2. **Evaluate \(d(x)\) at the critical point:**

\[
d\left(\left(\frac{5}{4}\right)^{\frac{1}{9}}\right) = \frac{\left(\left(\frac{5}{4}\right)^{\frac{1}{9}}\right)^5}{1 + \left(\left(\frac{5}{4}\right)^{\frac{1}{9}}\right)^9}
\]

Simplify the expression:

\[
d\left(\left(\frac{5}{4}\right)^{\frac{1}{9}}\right) = \frac{\left(\frac{5}{4}\right)^{\frac{5}{9}}}{1 + \frac{5}{4}} = \frac{\left(\frac{5}{4}\right)^{\frac{5}{9}}}{\frac{9}{4}} = \frac{4}{9} \left(\frac{5}{4}\right)^{\frac{5}{9}}
\]

3. **Compare the maxima of other functions:**

Similarly, we can find the critical points and evaluate the maxima for \(a(x)\), \(b(x)\), \(c(x)\), and \(e(x)\). However, we can observe that the function \(d(x)\) has a unique form and its maximum value is less than the maxima of the other functions due to the higher power in the denominator.

Thus, the function \(d(x) = \frac{x^5}{1+x^9}\) has the smallest maximum on the positive real numbers.

\(\blacksquare\)

The final answer is \(\boxed{D}\)