Why are RBCs efficient in their functions though they lack nucleus and mitochondria?

1. The ABSENCE OF A NUCLEUS leaves more room for HAEMOGLOBIN, so each cell carries more oxygen, and allows the cell to take up its BICONCAVE shape, which increases the surface area for gaseous exchange and lets it bend through narrow capillaries.
2. The ABSENCE OF MITOCHONDRIA means the cell does not respire aerobically, so it CONSUMES NONE OF THE OXYGEN it is carrying — all of it is delivered to the tissues.

The question asks why the cell is efficient DESPITE these absences, so the answer must show that each absence is actually an ADVANTAGE. That reframing is the whole point.

The mitochondria argument is the more elegant and is 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. Having no mitochondria, the red cell respires anaerobically and delivers its entire load.

THE COST of this specialisation, worth a closing line: with no nucleus the cell cannot repair itself or divide, so it lives only about 120 days and is then broken down in the spleen — the "graveyard of RBCs".