Hello Hannah,
Thank you for your inquiry and I m happy to assist you with this.
In order to be able to use the Opcode caching method (APC/APCu for example ), you need to make sure that the PHP extension is installed on your server. Only then you will be able to choose it from the Page Cache Method: list.
However, I would advise keeping the Page Cache Method: on Disk: Enhanced as it providing the best results when it comes to Page Caching.
Thanks!