Correct option is D
The length of the slot weld can be determined by the ratio of the load to the allowable stress. This relationship is derived from the basic equation of stress, which is stress = force / area. Rearranging this equation, the required weld area (and consequently the length of the weld) can be calculated by dividing the applied load by the allowable stress of the material.
Thus, the correct ratio for determining the length of a slot weld is load to allowable stress.