The correct answer is C) reduce the swinging of the lifeboat at the embarkation level. Frapping lines are used to reduce the swinging motion of a lifeboat at the embarkation level, preventing it from moving excessively and potentially causing injury to the occupants during the lowering process. This is an important safety measure required by maritime regulations. The other options are incorrect because they do not accurately describe the primary purpose of frapping lines. Option A relates to securing the lifeboat in the stowed position, option B describes a safety line for the occupants, and option D refers to bringing the lifeboat alongside the rail, which are not the main functions of frapping lines.