Conclusion :- "museums could relax their standards and save money without endangering their Renaissance oil paintings".
Option D :-
"None of the materials in Renaissance oil paintings other than the paint are vulnerable enough to relatively wide fluctuations in temperature and humidity to cause damage to the paintings".
Negating D :-
"Some of the materials in Renaissance oil paintings other than the paint are vulnerable enough to relatively wide fluctuations in temperature and humidity to cause damage to the paintings".{So, If there are some materials that are vulnerable enough and can destroy painting then obviously museums could not relax with their standards and save money invalidating the conclusion}.
A. Renaissance paintings were created in conditions involving far greater fluctuations in temperature and humidity than those permitted by current standards.
Incorrect : Comparison between conditions in which painting were made and painting are stored only helps to strengthen the argument but it is certainly not an assumption.
B. Under the current standards that museums use when storing Renaissance oil paintings, those paintings do not deteriorate at all.
Incorrect : If this answer choice is negated it does not weaken the argument. Eliminated.
C. Museum collections typically do not contain items that are more likely to be vulnerable to fluctuations in temperature and humidity than Renaissance oil paintings.
Incorrect : Issue is only about renaissance oil paintings. Other items in the museum are irrelevant to the scope of the argument.
D. None of the materials in Renaissance oil paintings other than the paint are vulnerable enough to relatively wide fluctuations in temperature and humidity to cause damage to the paintings.
Correct : The author takes into account only OIL PAINT in OIL PAINTINGS to come to the conclusion that museums should relax its standards for control of tempt. and humidity. The author fails to take into account other things (frame, canvas etc) that also are a part of a painting which needs protection. This is the underlying assumption on which the argument depends.
E. Most Renaissance oil paintings are stored in museums located in regions near the regions where the paintings were created.
Incorrect : Regions where paintings were created and paintings are stored is clearly irrelevant here.