Sunny3897 I think there is a small logical jump in your reasoning.
The question is asking-
Which bodily process is in danger of being accidentally disrupted by COX-2 inhibitors?
Your argument for choice A is that COX-2 inhibitors reduce PGI2 (which dilates blood vessels) while not preventing thromboxane production (which constricts blood vessels). That part is correct.
However, the passage never says that the process of blood vessel constriction is being disrupted. In fact, thromboxane is still being produced, so constriction can still occur.
For choice D, we have more direct support from the passage.
The first paragraph says that prostaglandins help in-
"controlling blood flow to the kidneys"The passage then explains that researchers were aware that blocking COX enzymes could stop production of prostaglandins that are not responsible for pain. Therefore, if COX-2 inhibitors affect substances beyond those causing pain and inflammation, kidney-related functions are potentially at risk.
So D follows directly from the passage, whereas A requires us to make an additional assumption about the overall effect on blood vessel constriction.
In RC, when one answer is directly supported and another requires an extra inference, we should prefer the directly supported choice.
Hope this helps!

Sunny3897
I don’t quite agree with the solution.
I don't agree with the solution. There is only a possibility that kidney function might be impacted but for option 1, we have double support. First, COX-2 inhibit PGI2, which dilates the blood vessels, and second, it does not prevent the production of Thromboxane, which constricts the blood vessels. Without the opposing effect of PGI2, it is more likely that blood vessels will be constricted.