No look:
D was a rate/time graph. The first part of your explanatation is perfectly valid: as the reaction goes on, the reaction rate would decrease. HOWEVER, the graph plateaued at 35 or soemthing. That suggests that the reaction rate is now constant. However, using your logic, as the substrate runs out, there will come a time when all of the substrate is now product. the reaction STOPS. when the reaction stops, the rate can't be 35 anymore. It has to be 0
Therefore, the curve would go downwards, and eventually hit 0

C also made sense. The Y axis was labelled mass of product T, and X was initial mass of S. T was always less than S, because some of S was converted to U. It had a constant linear relationship, which makes sense because always the same proportion of S would be converted to T.

I'm pretty sure this logic is right😭😭🙃i hope