Which of the following blood cells are flattened disc shaped that makes it suitable to carry oxygen from lungs to the tissues?

Erythrocytes also known as red blood cells (RBCs) function to transport oxygen in the blood. Mammalian erythrocytes are typically shaped as biconcave disks that is flattened and depressed in the center, with a dumbbell shaped cross section and a torus shaped rim on the edge of the disk. Oxygen transport is efficient because the cells have a flattened disc shape to increase surface area allowing rapid diffusion of oxygen and they contain haemoglobin which absorbs oxygen in the lungs and releases oxygen in the rest of the body.