What is the primary support structure running along the bottom of the ship?

The keel is the primary support structure that runs along the bottom of a ship, serving as the main backbone of the vessel. It is a critical component in shipbuilding, providing structural integrity and stability. The keel extends from the bow to the stern and is typically the first part of the ship to be constructed. It helps to maintain the shape of the hull and supports the weight of the ship and its cargo, acting as a pivotal element for balance and buoyancy.

While the hull refers to the overall outer structure of the ship that includes the keel, it is not specific to the bottom support. The galley is the kitchen area on a ship and does not relate to structural support. The waterline is the mark on the hull that indicates where the ship sits in the water, which plays a role in buoyancy but is not a structural component. Thus, the keel is uniquely identified as the primary support structure at the bottom of the ship, making it the correct answer.