Each of the following, if true, would weaken the conclusion that the whereabouts of the world's most valuable artwork is probably unknown
A) The value of a piece of art is purely subjective. CORRECT, this does not weaken the conclusion.
B) Serious art collectors usually publicize their new artwork several months after making a purchase. Hence the location is know if the collector publicize his new purcheses.
C) Museums own the vast majority of the world's most valuable artwork. If this is true, than the location of the most valuable work is know: the museums.
D) Of all the individuals in the world who own extremely valuable artwork, only a few are considered serious art collectors. If this is true, (the number of serious art collectors is little) then the number of valuable work whose whereabouts is unknown is little (it's reasonable to conclude so).
E) The private collections of most serious individual art collectors are often displayed in public settings.If they display them in public, then the location is know.
For me it was between A and D. If we analyze D closely, we can assume something that weakens the conclusion, but the info contained in A does not allow us to infer anything.