What is a good rule of thumb for the length of the anchor line?

Using an anchor line that is 7-10 times the depth of the water is a good rule of thumb because it ensures that the anchor can hold securely in varying conditions, accounting for factors like wind and current. When you use this ratio, it allows the anchor to set properly on the bottom, providing the necessary holding power while also minimizing the risk of the boat drifting. This length helps to create a more horizontal pull on the anchor, reducing the chances of it dragging along the bottom in choppy waters.

The other options suggest shorter lengths relative to the depth, which may not provide sufficient scope, risking instability and inadequate anchoring in more dynamic environments. It's important for boaters to adhere to this guideline to ensure safety and stability while at anchor.