Consider all quadrilaterals $ABCD$ such that $AB=14$, $BC=9$, $CD=7$, $DA=12$.  What is the radius of the largest possible circle that fits inside or on the boundary of such a quadrilateral?

$ \textbf{(A)}\ \sqrt{15} \qquad\textbf{(B)}\ \sqrt{21} \qquad\textbf{(C)}\ 2\sqrt{6} \qquad\textbf{(D)}\ 5\qquad\textbf{(E)}\ 2\sqrt{7} $
1. **Verify the Existence of an Incircle:**
   By Pitot's Theorem, a quadrilateral has an incircle if and only if the sum of the lengths of its opposite sides are equal. We need to check if \(AB + CD = AD + BC\).

   \[
   AB + CD = 14 + 7 = 21
   \]
   \[
   AD + BC = 12 + 9 = 21
   \]

   Since \(AB + CD = AD + BC\), the quadrilateral \(ABCD\) has an incircle.

2. **Set Up the Problem:**
   Let the circle be tangent to \(AB\), \(BC\), \(CD\), and \(DA\) at points \(P\), \(Q\), \(R\), and \(S\), respectively. Let the radius of the circle be \(r\) and the distances from the vertices to the points of tangency be \(x\), \(y\), \(z\), and \(w\).

3. **Express the Side Lengths in Terms of Tangent Segments:**
   \[
   AP = AS = x, \quad BP = BQ = y, \quad CQ = CR = z, \quad DR = DS = w
   \]
   \[
   AB = x + y = 14
   \]
   \[
   BC = y + z = 9
   \]
   \[
   CD = z + w = 7
   \]
   \[
   DA = w + x = 12
   \]

4. **Solve for the Tangent Segments:**
   Adding all the equations:
   \[
   (x + y) + (y + z) + (z + w) + (w + x) = 14 + 9 + 7 + 12
   \]
   \[
   2(x + y + z + w) = 42
   \]
   \[
   x + y + z + w = 21
   \]

   Using the individual equations:
   \[
   x + y = 14
   \]
   \[
   y + z = 9
   \]
   \[
   z + w = 7
   \]
   \[
   w + x = 12
   \]

   Subtracting the first equation from the sum:
   \[
   (x + y + z + w) - (x + y) = 21 - 14
   \]
   \[
   z + w = 7
   \]

   Subtracting the second equation from the sum:
   \[
   (x + y + z + w) - (y + z) = 21 - 9
   \]
   \[
   x + w = 12
   \]

   Subtracting the third equation from the sum:
   \[
   (x + y + z + w) - (z + w) = 21 - 7
   \]
   \[
   x + y = 14
   \]

   Subtracting the fourth equation from the sum:
   \[
   (x + y + z + w) - (w + x) = 21 - 12
   \]
   \[
   y + z = 9
   \]

5. **Calculate the Radius:**
   The area \(A\) of the quadrilateral can be found using Brahmagupta's formula for a cyclic quadrilateral:
   \[
   A = \sqrt{(s-a)(s-b)(s-c)(s-d)}
   \]
   where \(s\) is the semiperimeter:
   \[
   s = \frac{AB + BC + CD + DA}{2} = \frac{14 + 9 + 7 + 12}{2} = 21
   \]

   Substituting the side lengths:
   \[
   A = \sqrt{(21-14)(21-9)(21-7)(21-12)} = \sqrt{7 \cdot 12 \cdot 14 \cdot 9}
   \]
   \[
   A = \sqrt{7 \cdot 12 \cdot 14 \cdot 9} = \sqrt{10584}
   \]

   The radius \(r\) of the incircle is given by:
   \[
   r = \frac{A}{s} = \frac{\sqrt{10584}}{21}
   \]

   Simplifying:
   \[
   r = \frac{\sqrt{24 \cdot 441}}{21} = \frac{21 \sqrt{24}}{21} = \sqrt{24} = 2\sqrt{6}
   \]

   Therefore, the radius of the largest possible circle that fits inside or on the boundary of such a quadrilateral is \(2\sqrt{6}\).

The final answer is \(\boxed{2\sqrt{6}}\)