A vessel aground must exhibit __________ during the day.

A vessel that is aground must exhibit two black balls during daylight hours as a means of signaling its status. This is a requirement set forth by the International Regulations for Preventing Collisions at Sea (COLREGs) to ensure that other vessels are aware of the grounded vessel's situation. The two black balls are a recognized day shape that indicates the vessel is unable to navigate effectively due to being stuck on the bottom, making it crucial for nearby vessels to exercise caution to avoid collisions.

The other options would not properly convey the status of a grounded vessel. For example, a single black ball would not convey sufficient information about the vessel's situation, and a red flag is not an appropriate signal for this circumstance. Not displaying any signal would fail to alert nearby mariners, potentially leading to dangerous situations. Thus, the correct answer highlights the importance of signaling effectively to maintain navigational safety.