The blood leaving the tissues becomes richer in:
Simply, try to understand, when the oxygenated blood passes through the capillaries of the tissue, it gives oxygen to the body cells and takes carbon dioxide, produced during cellular respiration. Thus, it becomes richer in carbon dioxide.
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[A] Carbon dioxide
(A) carbon dioxide
You must know that, in human heart, the anterior vena cava collects, deoxygenated blood from the head, chest, and arms and enters the right atrium while the inferior vena cava collects blood from the lower body regions. Both venae cava pass the deoxygenated blood to the right atrium and, blood from tissues is rich in CO₂. You should choose option [A] as an answer.