What serves as a spacer to separate the flags in a hoist for clear reading of signals?

In maritime signal communication, a tail line serves a specific purpose in flag hoisting systems. It is used to create a gap or space between flags when they are hoisted, ensuring that each flag is clearly legible. When flags are hoisted without adequate spacing, they can overlap or tangle, which can lead to misinterpretation of signals. The tail line effectively separates these flags, allowing for an unobstructed view, which is essential for effective and clear communication at sea.

The other options do not fulfill this role in the same way. For example, a flag line typically refers to the line from which flags are actually hung, while signal lines might be used to indicate a specific type of communication rather than functioning as a spacer. The term "spacer line" is not commonly used in maritime practice, making it less relevant than the established principles of using a tail line for spacing.