Step 1: Understanding the poem reference.
The question refers to the famous fable-like poem where a mother duck cares for her ducklings and there is mention of a swan — specifically a “father swan.” Step 2: Analyze the options. (A) a sister: Doesn’t fit with the context of the poem. Incorrect. (B) a brother: Also unrelated to the story of the mother duck. Incorrect. (C) a father: Correct — The poem connects a mother duck with a father swan. (D) a grandfather: No mention of grandfather in the context. Incorrect. Step 3: Conclusion.
Thus, the correct answer is (C) a father.