A mature mammalian RBC does not contain a nucleus, endoplasmic reticulum, ribosomes, and mitochondria.

• The membrane-bound cell organelles are absent from adult mammalian RBCs. The biconcave, round, nonnucleated cell is their RBC.
• They have a short lifespan and are unable to produce new proteins or RNA. Mammalian mature red blood cells circulate in the body for around 100–120 days before being eliminated by the spleen. 
• At erythropoiesis phase three, it starts to lose mitochondria.
• Since the endoplasmic reticulum lowers the surface-to-volume ratio, hence its absence can allow RBC to become more flexible to allow them to squeeze into the capillaries.
• The RBC initially has a nucleus, but as it matures, it sheds it, improving the cell's ability to transport oxygen.

Hence, option D is the correct answer.