Cat C: Far more relaxed. Prisoners can wear their own clothes. Searches are infrequent. Outgoing mail can be sealed before submitting it. Incoming mail is opened in the presence of the prisoner (so that it can be checked for contraband but so that the prisoner can see that it's not read by staff). Some prisoners can acquire certain privileges, such as a single cell and the right to rent a TV set. Prisoners can also possess some items which are prohibited in Cat A & Cat B prisons. (e.g. prisoners can buy jars of coffee. They're prohibited in Cat A & B because the glass could be used as a weapon). Some prisoners might be permitted to leave the site to, for example, attend college or gain work experience. Prisoners are able to order their meals from a weekly menu, ensuring that they always get their choice of food. Prisoners spend most of their time out of their cells.
Cat D: 'Open prisons'. Even more relaxed, with greater opportunities for single accommodation, privileges and working or studying away from the prison.
Having got that lot out of the way (and with the caveat that no two prisons, even within the same category, are exactly alike), I'll try to answer your actual question: