Correct option (c) 12
Explanation :
In C4 -plants every CO2 molecule has to be fixed twice, so these plants are needed more energy for the synthesis of hexose sugar molecules than C3 -plants in which CO2 has to be fixed only once. 18 ATP mclecules are required by C3-plants for the synthesis of one molecule of hexose sugar while 30 ATP molecules are needed by the C4 -plants for the same. Thus, C4 -plants have a need of 12 ATP molecules extra than C3 -plants for the synthesis of one molecule of hexose sugar.