The correct answer is (C).
Getting to the Answer: When a question asks for an inference, the correct answer is something that can be deduced from the text, while not being directly stated. In the passage, the narrator states that Lydia never used a mirror after swimming but seemed "born to the sea, a kind of aquatic heiress," which suggests she looked elegant and in her element after swimming.
(A) Incorrect: the narrator is not talking about any regrets in these lines
(B) Incorrect: although the narrator states that Lydia does not use a mirror after swimming, nothing is said about other time spent grooming
(C) Correct: the narrator suggests that Lydia looks natural after swimming, suggesting a certain elegance
(D) the narrator does not provide evidence that Lydia was acting "overly formal"