Step 1: Understanding the Diagram.
This question involves identifying the structure in the diagram that is marked as "%".
Step 2: Analyzing the Options.
(A) Lymph nodes: Lymph nodes are part of the immune system but do not correspond to the marked structure.
(B) Heart: The heart is the central organ in the circulatory system, but this is not what is shown in the diagram.
(C) Lungs: Correct — The lungs are shown in the diagram as part of the respiratory system.
(D) Spleen: The spleen is part of the lymphatic system but does not match the diagram.
Step 3: Conclusion.
The correct answer is (C) Lungs, as the structure in the diagram corresponds to the lungs.