What is the observable feature at the 15 mark on the lead line?

The correct answer identifies the observable feature at the 15 mark on the lead line as a white rag. In nautical practice, lead lines are used to measure the depth of water and often feature distinctive markers at regular intervals to indicate specific depths.

The white rag is intentionally used as a clear visual indicator to denote the 15 fathom mark, providing an easily recognizable reference for mariners to quickly ascertain depth. This is crucial for navigation, ensuring vessels can safely operate in varying depths without the risk of grounding or running aground in shallow areas.

The other options do refer to various markers that might be encountered along other marks on the lead line, but they serve to denote different depths. Understanding the use of specific colors and knots to indicate particular measurements is essential in effective seamanship and ensuring safe navigation at sea.