The given rectangle has vertices \( A(0, 0) \), \( B(8, 0) \), \( C(8, 6) \), and \( D(0, 6) \).
  
 To find the length of the diagonal, we use the distance formula between any two opposite vertices of the rectangle.
  
 Let’s calculate the diagonal from \( A(0, 0) \) to \( C(8, 6) \).
  
 The distance formula is:
  \[
  d = \sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2}
  \]
  
 Substituting the coordinates of \( A(0, 0) \) and \( C(8, 6) \):
  \[
  d = \sqrt{(8 - 0)^2 + (6 - 0)^2} = \sqrt{8^2 + 6^2} = \sqrt{64 + 36} = \sqrt{100} = 10
  \]
  
 Therefore, the length of the diagonal is 10 units.