A. production of ammonia
B. manufacture of cement
C. photosynthesis
D. combustion
Correct Answer: Option C
C. photosynthesis
Explanation
Photosynthesis is the process of food-making in plants, it doesn’t have any by-products that pose a threat to the environment. Infact, the plants use our waste product of respiration (Carbon dioxide) and uses it for the photosynthetic process and gives off oxygen as by-product.