Could anyone please explain the approach to negate a double negative sentence...
Thank you in advance.

Negating statement C i.e.
The group of inmates who chose to take college-level courses were already less likely than other inmates to commit crimes after being released. This makes the conclusion that these inmates commit crimes after being released invalid. The reason is that these inmates are already less likely to commit crimes.

So, C it is.

Newspaper editorial: In an attempt to reduce the crime rate, the governor is getting tough on criminals and making prison conditions harsher. Part of this effort has been to deny inmates the access they formerly had to college-level courses. However, this action is clearly counter to the governor’s ultimate goal, since after being released from prison, inmates who had taken such courses committed far fewer crimes overall than other inmates.

Which of the following is an assumption on which the argument depends?

(A) Not being able to take college-level courses while in prison is unlikely to deter anyone from a crime that he or she might otherwise have committed.

(B) Former inmates are no more likely to commit crimes than are members of the general population.

(C) The group of inmates who chose to take college-level courses were not already less likely than other inmates to commit crimes after being released.

(D) Taking high school level courses in prison has less effect on an inmate’s subsequent behavior than taking college-level courses does.

(E) The governor’s ultimate goal actually is to gain popularity by convincing people that something effective is being done about crime.