Bundle sheath cells
C4 plants have a special kind of anatomy called Kranz anatomy, in which the bundle sheath cells form several layers around the vascular bundles. The mesophyll cells further form layers around the bundle sheath cells.
They are characterised by thick walls, no intercellular spaces and a large number of chloroplasts.
The thick walls and no intercellular spaces between the bundle sheath cells prevent the entry of oxygen and the escape of CO2.
Final answer
(C) Have a high density of chloroplasts