To find the length of the diagonal that divides a rectangle into two congruent right triangles, we can use the Pythagorean theorem.
In this case, the rectangle has a length of 16 units and a width of 12 units. The diagonal \(d\) can be calculated using the formula:
\[ d = \sqrt{(length^2) + (width^2)} \]
Substituting the given dimensions:
\[ d = \sqrt{(16^2) + (12^2)} \] \[ d = \sqrt{(256) + (144)} \] \[ d = \sqrt{400} \] \[ d = 20 \]
Therefore, the length of the diagonal cut she must make to divide the board is:
C. 20 inches