Which of the following works features the theme of endless longing?

The correct choice is "Out of the Cradle Endlessly Rocking," which is a poem by Walt Whitman. This work embodies the theme of endless longing through its exploration of loss, memory, and the deep yearning for connection. The poem reflects on the speaker’s experience of mourning the loss of a loved one, symbolized by the death of a young bird. The imagery of the cradle and the connectedness to nature enhances the feeling of nostalgia and the bittersweet nature of longing. Whitman’s use of vivid sensory details and rich emotional resonance creates a powerful sense of yearning that permeates the poem, illustrating a universal human experience of seeking something or someone that is lost.

The other options, while significant works in their own right, address different themes. "I Sing the Body Electric" mostly celebrates the human body and the beauty of life, "Calamus" delves into themes of companionship and fraternity, and "O Captain! My Captain!" focuses on mourning a leader after a significant victory, specifically addressing the themes of grief and admiration for Abraham Lincoln. None of those encapsulates the persistent sense of longing in the same way as "Out of the Cradle Endlessly Rocking."