What is the term used for the free end of a line on a boat?

The term used for the free end of a line on a boat is indeed referred to as the bitter end. In nautical terminology, the bitter end is the very end of a rope or line that is not secured to anything else. This terminology is crucial in boat handling and seamanship because it helps mariners describe and understand their equipment and actions with precision.

The remaining choices denote other parts of a line or different concepts related to nautical terms. The standing part refers to the portion of a line that is secured to an object, such as a cleat or winch, and not the free end. The working line denotes the section of the line that is actively in use for maneuvers and can either be the standing part or the bitter end depending on the context. An anchor line specifically refers to the line used to secure an anchor and does not define the concept of the free end of any line in general use. Therefore, the term bitter end accurately captures the specific meaning requested in the question.