Mature erythrocytes in humans lack nucleus and mitochondria.

1. The ABSENCE OF A NUCLEUS leaves more SPACE for HAEMOGLOBIN, so each cell carries more oxygen, and it allows the BICONCAVE shape which increases surface area and lets the cell bend through narrow capillaries. The nucleus is not needed, since a mature RBC does not divide.
2. The ABSENCE OF MITOCHONDRIA means the cell does not respire aerobically, so it CONSUMES NONE OF THE OXYGEN it is carrying — the entire load is delivered to the tissues.

The question asks why the cell lacks them, so each absence must be shown to be an ADVANTAGE rather than a deficiency. That reframing is the whole point.

THE MITOCHONDRIA ARGUMENT is the most elegant and the one candidates miss: a cell whose sole job is to TRANSPORT oxygen would be a poor carrier if it burned some of the cargo on the way.

THE COST of this specialisation: with no nucleus the cell cannot repair itself or divide, so it lives only about 120 days and is then destroyed in the SPLEEN — the "graveyard of RBCs".