This is from a Singapore-Cambridge GCE O-level Biology exam (a national exam for 16-year-olds). The exam board maintains that the question is "correct and accurate"—but isn't there a typo in option C? Here's my argument:

• Merely increasing the light intensity at either Points 1 or 2 speeds up photosynthesis, but varying the light intensity at Points 3 and 4 doesn't.

Therefore, light intensity is limiting photosynthesis at only Points 1 & 2.

• At Point 3, merely doubling the $$\ce{CO2}$$ concentration speeds up photosynthesis; on the other hand, the graph shows no evidence that varying the $$\ce{CO2}$$ concentrations at Points 1, 2 or 4 speeds up photosynthesis.

Therefore, $$\ce{CO2}$$ concentration is limiting photosynthesis conclusively at only Point 3.

As this is a data-inference question, all the four answer choices are invalid deductions.

(Asserting that $$\ce{CO2}$$ concentration is a limiting factor at Point 4 requires assuming that raising $$\ce{CO2}$$ concentration beyond 0.08% continues speeding up photosynthesis.)

Excluding Point 4 from Option C will render this question finally answerable.

• With your logic wouldn't that also mean that point 3 requires assuming that decreasing the carbon dioxide concentration would decrease photosynthesis rate? In statistics when you want to test if one parameter affects another it's sufficient to test a high and a low point.
– Cell
Aug 24, 2020 at 15:30
• @Cell No--because it stands to reason that $\ce{CO2}$ concentration is limiting photosynthesis in a given setup if varying it (in either direction) speeds up photosynthesis.
– user61462
Aug 24, 2020 at 18:00
• @Cell $\ce{CO2}$ concentration is said to be limiting photosynthesis in a given setup as long as increasing (or decreasing, for that matter) it there speeds up photosynthesis. In other words, the fact that increasing $\ce{CO2}$ at point 3 increases photosynthesis is sufficient to assert that $\ce{CO2}$ is a limiting factor there -- regardless of whether decreasing $\ce{CO2}$ there increases or decreases photosynthesis.
– user61462
Aug 24, 2020 at 22:10
• So, why do you think that logic doesn't apply at point 4? Aug 24, 2020 at 23:24
• @tyersome The graph displays no faster photosynthesis than at point 4.
– user61462
Aug 25, 2020 at 0:53

• 1. Without knowing that the optimal $CO_2$ concentration is above $0.08\%,$ one might guess that at point $4,$ $CO_2$ is a limiting factor, but one can't be certain. 2. At room temperature, the optimal $CO_2$ concentration for photosynthesis is actually $0.10\%.$ So, a $0.12\%\: CO_2$ graph would be no higher than—in fact, the same graph as—a $0.10\% \:CO_2$ graph. Nov 4, 2021 at 18:35