What is primarily used for indicating the working load on a mooring line?

The primary tool used for indicating the working load on a mooring line is the tattletale. This device provides a visual indication of the tension in the mooring line, which is crucial for safe mooring operations. By displaying the load on the line, it helps in ensuring that the mooring lines are not overloaded, thereby preventing failures that could lead to accidents or loss of vessel control. The tattletale is designed to give an early warning to crew members if the line is approaching its maximum allowable load, allowing for timely adjustments and interventions.

Understanding the function of the other options helps highlight why the tattletale is essential. A hawser is a type of heavy line or rope used for mooring or towing, but it does not indicate the load on its own. A marlinspike is a tool used for splicing and working with ropes and lines but does not provide any data regarding load. A fid is a pointed tool also used in ropework for making splices and knots, and similarly does not indicate any load measurements. Thus, the tattletale stands out as the specific tool for load indication.