To solve this problem, we need to identify which organism does not produce oxygen during photosynthesis. Let's analyze each option:$\newline \bullet$Cycas:$\newline$ - Cycas is a genus of gymnosperms that perform oxygenic photosynthesis, producing oxygen as a byproduct.$\newline$ - Therefore, Cycas produces oxygen during photosynthesis.$\newline \bullet$Nostoc:$\newline$ - Nostoc is a genus of cyanobacteria that also perform oxygenic photosynthesis, releasing oxygen.$\newline$ - Hence, Nostoc produces oxygen during photosynthesis.$\newline \bullet$Green sulphur bacteria:$\newline$ - Green sulphur bacteria are a group of photosynthetic bacteria that perform anoxygenic photosynthesis.$\newline$ - They use hydrogen sulfide (H2S) instead of water as an electron donor, and do not produce oxygen.$\newline$ - Therefore, green sulphur bacteria do not produce oxygen during photosynthesis.$\newline \bullet$Chara:$\newline$ - Chara is a genus of green algae that perform oxygenic photosynthesis, releasing oxygen.$\newline$ - Thus, Chara produces oxygen during photosynthesis.$\newline$Based on the analysis, the organism that does not produce oxygen during photosynthesis is:$\newline$Option 3: Green sulphur bacteria.$\newline$This corresponds to the correct option, which is Option 3.