Hey there!
I'm assuming it's pointing to the wall in between the two sections of the heart.
We can go over the function of the blue section and red section of the heart. Though I must admit I am not well versed with hearts of birds, I will assume this is similar to a human one. The blue section receives blood from the body which doesn't have any oxygen left in it, and the red section receives blood from the lungs, newly oxygenated. This sector in between separates these two sections, and separates oxygen-poor blood and oxygen-rich blood.
Thus, your answer is option D.
Hope this helps, feel free to let me know if you have any additional questions about this specific problem!