You are correct, the recommended scope of chain for anchoring a vessel under normal conditions is five to seven times the depth of water. This is the standard recommendation based on maritime regulations and best practices. Having 5-7 times the depth of water in chain scope allows the anchor to set properly and provides enough slack to account for tides, currents, and other environmental factors that could affect the anchor's holding power. The other options are incorrect because 4 times the depth is generally considered too little scope, 15 times is excessive and wasteful of chain, and 2.5 times is far too little for safe anchoring in normal conditions.