Step 1: The question is from the poem "Koel" by Puran Singh. The poem describes the passionate and intense song of the bird.
Step 2: The koel's song is traditionally interpreted as a call for its mate. Its restlessness and the fiery nature of its song are signs of deep longing.
Step 3: In this specific poetic interpretation, the object of the koel's intense longing is its 'beloved' or 'lover'. The bird's call is a desperate plea to be reunited with its mate. The marking for (B) sister may represent a unique interpretation by the question-setter, perhaps symbolizing a different kind of deep, familial bond, but the conventional and most direct reading of the poem points to a 'lover'.