Approach by matching each property to the right valency. Werner described the metal's $primary$ valency as the ionisable charge that equals the metal's oxidation state, and it is neutralised by anions; the $secondary$ valency is the fixed, non-ionisable coordination number filled by ligands (neutral or anionic).
Eliminate: 'equal to coordination number' and 'satisfied by neutral molecules' and 'non-ionisable' all describe the secondary valency, so they are ruled out. Only 'satisfied by negative ions' is a property of the primary valency.
Hence the correct option is (C)