This is understandably confusing. For one thing, a hinged door does not open through a “straight line” distance, but if I understand the meaning of the question and if both doors are the same size (width), which is a vital but omitted part of the question, then I presume that the measures of the arcs through which the doors open are 40” and 48”, respectively.
So, that’s my understanding of the “straight line distance” that’s mentioned (and if you consider how a narrow door will open through a smaller arc than a wider door – measured by the length of the arc at the outer edge of the door, then you’ll understand why it’s vital to compare doors of the same width), but the final question is also worded somewhat oddly.
The wording you mention is “which doorstop needs to be set to open to a larger angle”, which, again, I presume to mean “to permit a wider-opening door”. If you diagram this, I think the answer will be clear to you (provided you agree with my interpretations of the problem setup and what is being asked).