Prison gangs often engage in illegal economic activities such as within the prison walls.

ANSWER:

Prison gangs often engage in "illegal economic activities" like prostitution within the "prison walls".

EXPLANATION:

Prostitution helps the prison gangs to earn extra money in all illegal manner. Prison gangs generally do not get to earn a lot in prison walls. These gangs indulge in a variety of activities so that they can earn some money but in an illegal manner. The prison gangs also indulge in these illegal economic activities in a secret manner without letting the prison authorities know of the same.

C; skin, is the answer!