Final answer:
The set of red arrows labeled Fc and Fw, which indicate compressional forces acting horizontally towards the center of the rock, best represents the direction of maximum directional stress.
Step-by-step explanation:
To determine which set of arrows best represents the direction of maximum directional stress on a rock, we consider the types of stress being represented: compression, tension, and shear. The arrows labeled Fc and Fw, with magnitudes of 9.0 N and 13.0 N respectively, show forces acting horizontally towards the center, indicating compressional forces. Because rocks can typically withstand compressional stress more than tensional stress, and given no information about shear stress, the set of arrows with the larger magnitudes, Fc and Fw, would represent the maximum directional stress from what is provided in the question.