The correct answer is D) To reduce side slip of the vessel. A centerboard is a retractable keel that extends below the hull of a sailboat. When sailing on a tack, the centerboard's primary purpose is to reduce the vessel's side slip, or leeway, by providing lateral resistance. This helps keep the boat moving in the desired direction rather than drifting sideways. The centerboard acts as a wing underwater, generating lift and counteracting the forces that would otherwise cause the boat to slide off course. The other answer choices are not the main reasons for using a centerboard. While a centerboard can contribute to the vessel's stability and prevent unintended jibing, these are secondary benefits, not the primary purpose when sailing on a tack.