The correct answer is A) The vessel will steer a fairly straight course. When a vessel is equipped with twin propellers turning outboard with the engines at half ahead, and there is no wind or current with the rudders amidships, the opposing thrust from the two propellers will essentially cancel each other out, allowing the vessel to maintain a fairly straight course. This is because the outward thrust from the two propellers counteracts any tendency for the vessel to swing or steer to port or starboard. The other answer choices are incorrect because: B) and C) would require an asymmetrical thrust or unbalanced force to cause the bow to swing, which is not the case here; and D) a zigzag course would require some external factor, such as wind or current, to induce yaw or directional instability, which is not present in this scenario.