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.