Easy, but a bit messy to explain. I’ll try...
Remember respiration (produces CO2) and photosynthesis (uses up CO2) occur together.
When light intensity = 0 (point A on graph) there is no photosynthesis. The graph shows CO2 uptake= -10ml/min. So the plant must be *producing* CO2 at 10ml/min due to respiration.
We can assume this rate of respiration is constant, always 10ml/min.
When light intensity = 2 units (point B on graph) CO2 uptake is zero, so the plant is producing CO2 by respiration at the same rate as it photosynthesis uses it up. This tells us 2 units of light intensity causes photosynthesis to use CO2 at 10ml/min.
At point D, light intensity is 4 units so we know:
respiration: CO2 produced at 10ml/min.
photosyntheses CO2 used up at 2 x 10 = 20ml/min.
Overall CO2 uptake = 20 – 10 = 10ml/min, as shown at point D.
Since 20>10 the answer is:
“amount of CO2 used in photosynthesis by plant would be >10 ml/min.
You don’t actually have to do any calculations once you understand what’s going on. You can see staight away that going from point A to point D, photosynthesis must be using up CO2 at 20ml/min at D.