The `C_4` plants are twice as efficient as `C_3` plants in terms of fixing `CO_2` but lose only ... as much water `C_3` plants for the same amount of `CO_2` fixed.

Explanation

Half (less of water). The work efficiency of `C_4` is as such that in which `CO_2` fixation becomes maximum. The water loss in its comparisons less (half).
`C_4` plants fix `CO_2` and possess double work efficiency than `C_3` in formation of Glucose. But during equal `CO_2` fixation loses water half than `C_3`.