The correct answer is D) Green square. The U.S. Coast Guard's Aids to Navigation System uses a standardized system of daymarks, which are visual signs or markers placed along waterways to indicate the proper channel. When entering a channel from the sea, a green square daymark is used to mark the port (left) side of the channel. This is in accordance with the lateral buoyage system, where green markers indicate the left side of the channel when facing upstream or entering from the sea. The other options are incorrect because: A) A red and white octagon is used to mark the starboard (right) side of the channel, B) A red triangle is used to mark a special purpose marker, and C) A black and white diamond is used to mark an obstruction or other hazard in the waterway.