The correct answer is A) little or no horizontal motion of the water. Slack water occurs when there is little or no horizontal motion of the water, typically at the change between the flood (incoming) and ebb (outgoing) tides. At this point, the current is at its weakest and there is minimal horizontal water movement, resulting in slack water conditions. The other options are incorrect because: B) refers to wind-driven water backup, not slack water; C) describes a weak current, but does not specify the lack of horizontal motion; and D) refers to vertical water motion, which is not the defining characteristic of slack water.