The correct answer is A) cleat. A cleat is a deck fitting used to secure line or wire rope, consisting of a single body with two protruding horns. Cleats are a common and essential fitting on vessels, as they provide a secure point to tie off and secure lines, ropes, and cables. The two protruding horns allow the line to be wrapped around the cleat, providing a strong and reliable hold. The other options are incorrect because: B) a bollard is a single post-like fitting, C) a bitt is a pair of upright posts used for securing mooring lines, and D) a capstan is a vertical-axis winch used for hauling in or letting out a line.