In this poem by Ruskin Bond, the use of the word heart-beats suggests the narrator is amazed by the existence of life.
The answer is D.
Although the speaker names darkness and light and struggle as a condition, he does it as an inherent characteristic of life. He sounds amazed about how the hearts that inhabit the world survive all these situations in life. He speaks with hope and joy about everything around us; this Universe, this particular planet and its beautiful landscapes, the heart beating beings that we know and the ones we don´t.
It is a bright, positive look towards life.