The correct answer is A) Effect of the moon's gravity. In mid-ocean, the characteristics of a wave are determined by three factors: the length of time a wind has been blowing (duration), the wind velocity, and the fetch (the distance over which the wind has blown). The effect of the moon's gravity is not one of the three primary factors that determines wave characteristics in the open ocean. This is because wave formation is primarily driven by wind and atmospheric conditions, rather than gravitational forces. The other answer choices (B, C, and D) are all correct factors that influence wave characteristics in the open ocean.