The correct answer is D) 50 knots. Ocean swells originating from a typhoon can move ahead of the storm at speeds near 50 knots. This is because the swells are generated by the winds of the typhoon, which can reach speeds of 50 knots or more. As the swells propagate outward from the storm, they can travel at these high speeds, often arriving at distant locations before the typhoon itself. The other answer choices are incorrect because they underestimate the typical speed of these ocean swells. While 10, 20, or 30 knots may be reasonable speeds for smaller, more localized waves, the large swells generated by a powerful typhoon can far exceed those values.