$C_4$ plants are more evolved than  $C_3$ plants because

a) $C_4$ plants are twice efficient in terms of carbon fixing

b) $C_4$ plants loose only half of its water as $C_3$ plants for same amount of ${\mathrm{CO}}_2$ fixed.

c) $C_4$ plants loose only half of the amount of water as $C_3$ plants for double the amount of ${\mathrm{CO}}_2$ fixed.

 

The evolution of the C₄ photosynthetic system is probably one of the strategies for maximising the availability of CO₂ while minimising water
loss. C₄ plants are twice as efficient as C₃ plants in terms of fixing carbon dioxide (making sugar). However, a C₄ plant loses only half as much water as a C₃ plant for the same amount of CO₂ fixed.