Answer: D) She had been surfing for years, but these waves were the most monstrous she had ever seen.
Explanation: From the given options, the best sentence to finish the paragraph is the corresponding to option D: She had been surfing for years, but these waves were the most monstrous she had ever seen, because by saying that the waves were "monstrous" it continues the comparison of the waves to an angry sea creature (made by Tabitha to express how challenging will be the ride).