Assertion (A) – RBCs are circular and biconcave in shape lacking all cell organelles.
Reason (R) - Circular and biconcave shape increase the surface area for oxygen binding and the lack of organelles results in no self-utilisation of the oxygen being carried.
Both A and R are correct and R is the correct explanation of A.
RBCs are usually biconcave and circular in shape. This gives them more surface area resulting in efficient binding of oxygen molecules. Moreover, due to the lack of organelles, the RBCs cannot and need not utilise the oxygen they carry for themselves. This also gives them flexibility.