Which action would most likely be considered unconstitutional under the precedent established in the Regents of the University of California v. Bakke Supreme Court decision?

C is the correct answer.
In Regents v. Bakke, the Supreme Court held that racial quotas were impermissible but that schools were permitted to have race be one of many factors that were considered when colleges considered an applicant.
So, a 10 percent threshold would likely be found to be unconstitutional.