Final answer:
To gather ninety Steinberg paintings for an exhibit at Columbia University as described in "Outsider's Art Is Saluted at Columbia, Then Lost Anew" is B) The university had purchased the paintings.
Step-by-step explanation:
In the article "Outsider's Art Is Saluted at Columbia, Then Lost Anew," it is mentioned that the exhibit at Columbia University included ninety Steinberg paintings. These paintings were acquired by the university through purchase, as stated in the options.
According to the information provided, there is no mention of Steinberg donating the paintings to the university or former students keeping them since acquiring them. Additionally, the option of collecting the paintings from trash cans on move-out days is not mentioned as a means of gathering the artwork for the exhibit. Therefore, the correct answer is B) The university had purchased the paintings.