The correct option is C 600 : 1
White blood cells or leucocytes are fewer in number than the red blood cells, with a ratio of about 1 WBC to every 600 - 700 RBCs.
WBCs are responsible for defending the body against infections, while the RBCs carry oxygen from lungs and deliver to all parts of the body.
Thus, the correct answer is option (C), '600 : 1'.