i believe c to refers to “connect to” on the pcie adapter / card ur hoping to connect.
the Diagram is a literal display of the socket on the main board, the black areas are GND. The columns connected are 2,3,5,7,9,10,12,13,15,16
the rows are 1-9 so either 1,3,5,7,9 or 9,7,5,3,1 are gnd leaving...