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. The question is asking about the number of common 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), there are three common navigation lights that must be displayed from sunset to sunrise and during periods of restricted visibility such as fog. 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 arc of 225 degrees, visible from directly ahead to 67.5 degrees on either side of the beam.<br />## Step 3<br />These lights are essential for ensuring the safety of navigation and preventing collisions between vessels.