6. There are __ common navigation lights that must be displayed from sunset to sunrise and during periods of restricted visibility, such as fog. a two b three c four

## Step 1<br />This problem is related to maritime navigation and the rules and regulations governing it. Specifically, it is about the navigation lights that must be displayed from sunset to sunrise and during periods of restricted visibility, such as fog.<br />## Step 2<br />According to the International Regulations for Preventing Collisions at Sea (COLREGS), a vessel must display three navigation lights from sunset to sunrise and during periods of restricted visibility. These lights are:<br />1. A green light on the starboard side.<br />2. A red light on the port side.<br />3. A white light which shows an unbroken light over an extended period.<br />## Step 3<br />These lights are essential for the safety of navigation, as they help other vessels identify the type and position of the vessel, especially in low visibility conditions.