Competitive inhibitors resemble the substrate and bind to the active site of the enzyme. In the diagram, the substrate is shown as a triangular shape that fits into the V-shaped active site. Now compare the inhibitors: P → contains a triangular portion, resembling the substrate shape → can compete for the active site. S → diamond shape with pointed ends similar to the substrate geometry → also likely to fit the active site. Q, R, T → shapes do not resemble the substrate, so they are less likely to act as competitive inhibitors. Therefore, the best pair of competitive inhibitors is: P and S.