Final answer:
The true statement about mature, circulating erythrocytes is that they have no nucleus, which allows for more space for hemoglobin, rather than being packed with mitochondria or surviving for only 4 days.
Step-by-step explanation:
The correct answer is: a. They have no nucleus.
Mature, circulating erythrocytes, also known as red blood cells, are distinctive in that they lack a nucleus. This allows for more space within the cells for hemoglobin, the protein responsible for carrying oxygen. Contrary to the false options, erythrocytes do not contain mitochondria, as they generate their energy via glycolysis, a process that does not require mitochondria.
Additionally, erythrocytes have a lifespan considerably longer than 4 days; they typically survive for about 120 days in the circulatory system before being removed by the spleen. Thus, a statement claiming they are packed with mitochondria or that they survive for an average of 4 days is incorrect.