The correct answer is option 2: 150 days. The lifespan of red blood cells (RBCs) in a man is approximately 150 days. RBCs are responsible for carrying oxygen to all the cells in the body. They are constantly being produced in the bone marrow and old or damaged RBCs are continuously being removed from circulation and replaced with new ones.
Option 1, 20 days, is not correct as the lifespan of RBCs is much longer than that. Option 3, 180 days, and option 4, 190 days, are also not correct as they overestimate the lifespan of RBCs.
It is important to note that the lifespan of RBCs can vary slightly from person to person and can also be affected by certain medical conditions. However, on average, the lifespan of RBCs in a man is around 150 days.