For each vertex of a solid cube, consider the tetrahedron determined by the vertex and the midpoints of the three edges that meet at that vertex.  The portion of the cube that remains when these eight tetrahedra are cut away is called a [i]cuboctahedron[/i].  The ratio of the volume of the cuboctahedron to the volume of the original cube is closest to which of these?

$ \textbf{(A)}\ 75\%\qquad\textbf{(B)}\ 78\%\qquad\textbf{(C)}\ 81\%\qquad\textbf{(D)}\ 84\%\qquad\textbf{(E)}\ 87\% $
1. Let the side length of the cube be \( x \). The volume of the cube is given by:
   \[
   V_{\text{cube}} = x^3
   \]

2. Consider one of the tetrahedra formed by a vertex of the cube and the midpoints of the three edges meeting at that vertex. Each edge of the cube has length \( x \), so the length of each edge of the tetrahedron is \( \frac{x}{2} \).

3. The area of the triangular base of the tetrahedron can be calculated as follows. The base is a right triangle with legs of length \( \frac{x}{2} \):
   \[
   \text{Area of base} = \frac{1}{2} \left( \frac{x}{2} \right) \left( \frac{x}{2} \right) = \frac{x^2}{8}
   \]

4. The height of the tetrahedron is the distance from the vertex of the cube to the midpoint of the base, which is \( \frac{x}{2} \).

5. The volume of one tetrahedron is given by:
   \[
   V_{\text{tetrahedron}} = \frac{1}{3} \times \text{Area of base} \times \text{Height} = \frac{1}{3} \times \frac{x^2}{8} \times \frac{x}{2} = \frac{x^3}{48}
   \]

6. Since there are 8 such tetrahedra in the cube, the total volume of the tetrahedra is:
   \[
   V_{\text{total tetrahedra}} = 8 \times \frac{x^3}{48} = \frac{8x^3}{48} = \frac{x^3}{6}
   \]

7. The volume of the cuboctahedron is the volume of the cube minus the volume of the 8 tetrahedra:
   \[
   V_{\text{cuboctahedron}} = V_{\text{cube}} - V_{\text{total tetrahedra}} = x^3 - \frac{x^3}{6} = \frac{6x^3}{6} - \frac{x^3}{6} = \frac{5x^3}{6}
   \]

8. The ratio of the volume of the cuboctahedron to the volume of the original cube is:
   \[
   \text{Ratio} = \frac{V_{\text{cuboctahedron}}}{V_{\text{cube}}} = \frac{\frac{5x^3}{6}}{x^3} = \frac{5}{6} \approx 0.8333
   \]

9. Converting this ratio to a percentage:
   \[
   0.8333 \times 100\% = 83.33\%
   \]

10. The closest percentage to 83.33% from the given options is 84%.

The final answer is \(\boxed{84\%}\)