The blood leaving the tissues becomes richer in
(a) carbon dioxide
Blood leaving the tissue is enriched with carbon dioxide. The aorta carries oxygenated blood from the heart to the organs. When this blood passes through the capillaries of body organs, it gives oxygen to the body cells.
Hence the blood which reaches the tissue is enriched with oxygen which is absorbed by the tissue and the carbon dioxide from the cell is exchanged for oxygen.
Deoxygenated blood (carrying ) leaves the tissues and enters vena cava, from where they are again passed to the lungs for
Hence option (a) is correct.