A power-driven boat less than 7 meters and whose maximum speed does not exceed 7 KTS only has to show an _______ ______________ light.

In maritime navigation, the signaling requirements for vessels vary based on their size and type. For a power-driven boat that is less than 7 meters in length and has a maximum speed not exceeding 7 knots, the correct signal light to display is an all-round light. This is because the all-round light is designed to provide visibility from all directions, ensuring that other vessels can see the boat regardless of their approach angle.

This requirement falls under the International Regulations for Preventing Collisions at Sea (COLREGs), which outlines specific lighting and signaling protocols for different classes of vessels. The all-round light serves as a simple, effective means for smaller vessels to indicate their presence on the water, especially in low visibility conditions, while also complying with regulations.

The other types of lights mentioned, such as flashing, beacon, and steady lights, have specific purposes for different sizes and types of vessels but do not meet the particular requirements for this category of power-driven boats. Therefore, the use of an all-round light is both a regulatory requirement and a practical safety measure for smaller power-driven boats on the water.