Questions: What's the role of red blood cells? a) Immune defense against bacterial infections b) Transport of O₂ from the lungs c) Immune defense against viral infections d) Transport of albumin to muscle cells

The answer is the second one (b): Transport of $\mathrm{O}_{2}$ from the lungs.

Explanation for each option: a) Immune defense against bacterial infections - This is incorrect. The primary cells involved in immune defense against bacterial infections are white blood cells, such as neutrophils and macrophages. b) Transport of $\mathrm{O}_{2}$ from the lungs - This is correct. Red blood cells (erythrocytes) are responsible for carrying oxygen from the lungs to the rest of the body and returning carbon dioxide from the body to the lungs to be exhaled. c) Immune defense against viral infections - This is incorrect. Similar to bacterial infections, the immune defense against viral infections is primarily the role of white blood cells, such as lymphocytes. d) Transport of albumin to muscle cells - This is incorrect. Albumin is a protein produced by the liver and is transported in the blood plasma, not specifically by red blood cells.

Summary: The role of red blood cells is to transport oxygen ($\mathrm{O}_{2}$) from the lungs to the rest of the body.