Question

For the same amount of co2 fixed, a c4 plant in comparison with a c3 plant loses only

Answer 1

yes it's bcz cranz anatomy
here the enzyme cause binding of co2 hence

Answer 2

It is because of the kranz anatomy. Now I'm describing what is kranz anatomy.

Kranz anatomy is a special type of leaf structure which is present in the leaf of c4 plants and here the the tissue equivalent to the spongy mesophyll cells is clustered in a ring around the leaf veins, outside the bundle sheath cells which are present in the leaves of the plant. The bundle sheath cells contain large chloroplast and the mesophyll cell contain small chloroplast.