This statement about the dark reactions is true.

a. dark reactions occur in the thylakoids of chloroplasts.
b. light energy is converted to chemical energy during the dark reactions.
c. carbon dioxide is converted to sugar using atp and nadph during the dark reactions.
d. all of the above

The best and most correct answer among the choices provided by your question is the third choice or letter C. Dark reaction involves Carbon dioxide which is converted to sugar using ATP and NADPH during the dark reactions.

Dark reactions make use of these organic energy molecules (ATP and NADPH). This reaction cycle is also called Calvin Benison Cycle, and it occurs in the stroma. ATP provides the energy while NADPH provides the electrons required to fix the CO2(carbon dioxide) into carbohydrates.

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The statement that is true regarding the second major process of photosynthesis includes the fact that Carbon dioxide is fixed and or converted to sugar using ATP and NADPH during this cycle or series of reactions that occurs.