The correct answer is option 4, bone marrow. Red blood cells, also known as erythrocytes, are produced in the bone marrow. The bone marrow is a soft, spongy tissue found in the center of certain bones, such as the hips, thighs, and the breastbone. It is responsible for the production of various blood cells, including red blood cells.

Option 1, the liver, is not involved in the production of red blood cells. The liver has several important functions in the body, such as detoxification, digestion, and the production of certain proteins, but it does not produce red blood cells.

Option 2, voluntary muscles, are not involved in the production of red blood cells either. Voluntary muscles are the muscles that we can control, such as the muscles in our arms and legs. They are responsible for movement and do not play a role in blood cell production.

Option 3, the pancreas, is also not responsible for the production of red blood cells. The pancreas is an organ located near the stomach and plays a role in digestion and the regulation of blood sugar levels, but it does not produce blood cells.

In summary, red blood cells are produced in the bone marrow, which is the correct answer.