If the six solutions of $x^6=-64$ are written in the form $a+bi$, where $a$ and $b$ are real, then the product of those solutions with $a>0$ is

$\text{(A)} \ -2 \qquad \text{(B)} \ 0 \qquad \text{(C)} \ 2i \qquad \text{(D)} \ 4 \qquad \text{(E)} \ 16$
1. First, we need to find the six solutions to the equation \(x^6 = -64\). We can express \(-64\) in polar form as \(64e^{i\pi}\) because \(-64 = 64 \cdot e^{i\pi}\).

2. To find the sixth roots of \(64e^{i\pi}\), we use the formula for the \(n\)-th roots of a complex number. The \(k\)-th root is given by:
   \[
   x_k = \sqrt[6]{64} \cdot e^{i\left(\frac{\pi + 2k\pi}{6}\right)}
   \]
   where \(k = 0, 1, 2, 3, 4, 5\).

3. Calculate \(\sqrt[6]{64}\):
   \[
   \sqrt[6]{64} = 2
   \]

4. Now, we find the six roots by substituting \(k = 0, 1, 2, 3, 4, 5\) into the formula:
   \[
   x_k = 2 \cdot e^{i\left(\frac{\pi + 2k\pi}{6}\right)}
   \]

5. Simplify the exponents:
   \[
   x_0 = 2 \cdot e^{i\frac{\pi}{6}}
   \]
   \[
   x_1 = 2 \cdot e^{i\frac{\pi + 2\pi}{6}} = 2 \cdot e^{i\frac{3\pi}{6}} = 2 \cdot e^{i\frac{\pi}{2}}
   \]
   \[
   x_2 = 2 \cdot e^{i\frac{\pi + 4\pi}{6}} = 2 \cdot e^{i\frac{5\pi}{6}}
   \]
   \[
   x_3 = 2 \cdot e^{i\frac{\pi + 6\pi}{6}} = 2 \cdot e^{i\frac{7\pi}{6}}
   \]
   \[
   x_4 = 2 \cdot e^{i\frac{\pi + 8\pi}{6}} = 2 \cdot e^{i\frac{9\pi}{6}} = 2 \cdot e^{i\frac{3\pi}{2}}
   \]
   \[
   x_5 = 2 \cdot e^{i\frac{\pi + 10\pi}{6}} = 2 \cdot e^{i\frac{11\pi}{6}}
   \]

6. Convert these roots to rectangular form \(a + bi\):
   \[
   x_0 = 2 \left(\cos\frac{\pi}{6} + i\sin\frac{\pi}{6}\right) = 2 \left(\frac{\sqrt{3}}{2} + i\frac{1}{2}\right) = \sqrt{3} + i
   \]
   \[
   x_1 = 2 \left(\cos\frac{\pi}{2} + i\sin\frac{\pi}{2}\right) = 2 \left(0 + i\right) = 2i
   \]
   \[
   x_2 = 2 \left(\cos\frac{5\pi}{6} + i\sin\frac{5\pi}{6}\right) = 2 \left(-\frac{\sqrt{3}}{2} + i\frac{1}{2}\right) = -\sqrt{3} + i
   \]
   \[
   x_3 = 2 \left(\cos\frac{7\pi}{6} + i\sin\frac{7\pi}{6}\right) = 2 \left(-\frac{\sqrt{3}}{2} - i\frac{1}{2}\right) = -\sqrt{3} - i
   \]
   \[
   x_4 = 2 \left(\cos\frac{3\pi}{2} + i\sin\frac{3\pi}{2}\right) = 2 \left(0 - i\right) = -2i
   \]
   \[
   x_5 = 2 \left(\cos\frac{11\pi}{6} + i\sin\frac{11\pi}{6}\right) = 2 \left(\frac{\sqrt{3}}{2} - i\frac{1}{2}\right) = \sqrt{3} - i
   \]

7. Identify the roots with \(a > 0\):
   \[
   x_0 = \sqrt{3} + i
   \]
   \[
   x_5 = \sqrt{3} - i
   \]

8. Calculate the product of these roots:
   \[
   (\sqrt{3} + i)(\sqrt{3} - i) = (\sqrt{3})^2 - (i)^2 = 3 - (-1) = 3 + 1 = 4
   \]

The final answer is \(\boxed{4}\).