During the day, a vessel with a tow over 200 meters in length will show __________.

A vessel with a tow over 200 meters in length during the day will display a diamond shape as a signal. This signaling is in accordance with the International Regulations for Preventing Collisions at Sea (COLREGs) governing the behavior of vessels. The diamond shape signifies that the vessel is towing and indicates its size, which is particularly important for other vessels to be aware of in order to take appropriate navigational actions.

This signal helps in conveying to other mariners that the vessel's tow is significant in length, necessitating extra caution. The requirement for different shapes and signals based on tow lengths is critical for maritime safety, allowing for clear communication of a vessel's navigation status to avoid potential collisions.

The other possible signals, such as a rectangular shape or two mastheads, do not apply to a vessel with a tow of this length under such circumstances. A rectangular shape is typically reserved for different contexts, and two mastheads do not denote length but rather are used for signaling specific operations. No special signal is incorrect as vessels with long tows must signify their status to ensure safety on the waterway.