In the final paragraph, we learn that some plants have come up with a way to ensure that oxygen (O) doesn't interfere with photosynthesis.
This method involves separating: (1) where water is split into hydrogen (H) and O; and (2) where sugars are built from carbon dioxide (CO2).
The first process occurs in specialized chlorophyll whereas the second process occurs where rubisco is located.
However, we then learn that O and all other atmospheric gases are excluded from the areas where rubisco is located. This raises a problem. CO2 is an atmospheric gas and it works in conjunction with rubisco to make sugars. So, in plants which separate these processes, how does CO2 get to the rubisco?
This is explained starting at line 50. CO2 goes through a series of reactions to form a "nongas" called C-4. C-4 is allowed to enter the area where rubisco is located as it is not a gas. It then releases the CO2 which works with rubisco to make sugars.
So, the question stem asks us why the author refers to "all other atmospheric gases?" We can see from our analysis above that this is to introduce the problem of CO2 reaching the rubisco which is then explained by the creation of C-4.
This is restated by answer choice (D), making it the correct answer.
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