elephantsmall

Necessary assumption.  Use negation.

If you negate C, you get: The group of inmates who chose to take college-level courses WERE already less likely than other inmates to commit crimes after being released. If so, these group of inmate are GOOD inmates to begin with, with or without the college-level course training. Therefore, the argument that allowing an inmate to take such courses in prison would decrease an inmate's chance of committing crimes after his or her release is false.

Thus, C is the necessary assumption