The correct answer is C. 1.5.
The maximum shear stress in a rectangular section occurs at the neutral axis, and the average shear stress occurs at the centroid of the section. The ratio of the maximum to average shear stresses is given by the following equation:
$\frac{\tau_{max}}{\tau_{avg}} = \frac{3}{2}$
Therefore, the ratio of maximum to average shear stresses on a rectangular section is 1.5.
Option A is incorrect because the maximum shear stress is always greater than the average shear stress. Option B is incorrect because the ratio of maximum to average shear stresses is not equal to 1.25. Option D is incorrect because the ratio of maximum to average shear stresses is not equal to 2.5.