Which of the following best describes bitts?

Bitts are best described as pairs of heavy vertical cylinders used to secure lines, primarily on vessels. They serve as strong points where mooring lines, tow lines, and other types of rigging can be fastened securely. The design of bitts allows for effective load distribution, which is crucial for maintaining the stability and safety of a ship when docked or when securing various equipment.

The other options, while related to vessel operation, do not accurately define bitts. A fitting for leading mooring lines denotes equipment involved in guiding and handling lines but does not capture the specific function and structure of bitts. Light-weighted lines for throwing are typically associated with tasks like heaving lines or messenger lines and do not refer to the robust construction of bitts. Lastly, a capstan is a different type of device used for powerful winding and hoisting operations, distinct from the function of bitts. Overall, the unique utility and physical characteristics of bitts solidly align them with the first choice.