Use the overall photosynthesis equation

The simplified overall reaction for formation of one molecule of glucose is:

$6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$

Recall ATP requirement in the Calvin cycle

In the Calvin cycle, fixation of one molecule of $CO_2$ requires:

$3\,ATP$

because for every $3CO_2$ converted to one net molecule of glyceraldehyde-3-phosphate (G3P), the cycle consumes:

$9\,ATP$

Glucose formation requires 6 molecules of $CO_2$

One glucose molecule is formed from:

$6CO_2$

Therefore, ATP required is:

$6 \times 3 = 18\,ATP$

Check the options

• A: $10$ ATP — incorrect
• B: $8$ ATP — incorrect
• C: $6$ ATP — incorrect
• D: $18$ ATP — correct

Final answer

Synthesis of each molecule of glucose in photosynthesis involves:

$\boxed{18\text{ molecules of ATP}}$