Which mark shows that there are two knots tied on the lead line?

The correct answer is the 20 mark. In maritime practice, marks on the lead line indicate the depth and can also signify knots tied along the line. Specifically, the 20 mark signifies that there are two knots tied. Each knot represents a specific depth, and depths are usually measured in fathoms or feet depending on the context of use.

Understanding how to read a lead line correctly is crucial for seamanship, as it provides valuable information about underwater conditions that affect navigational decisions. Knowing the significance of the markings on the lead line enables mariners to assess depth and make informed choices during operations such as anchoring, fishing, and navigating in shallow waters.