Step 1: Recall Werner's theory: two types of valency.
According to Werner, a central metal atom in a coordination compound has two types of valencies: (i) primary (principal) valency, corresponding to the oxidation state, and (ii) secondary (auxiliary) valency, corresponding to the coordination number.
Step 2: Understand primary valency in detail.
Primary valency corresponds to the oxidation state (oxidation number) of the central metal. It is ionisable: the groups satisfying primary valency exist outside the coordination sphere and ionise freely in solution.
Step 3: What types of groups satisfy primary valency?
Primary valency is satisfied exclusively by negative ions (anions). For example, in $[Co(NH_3)_6]Cl_3$, the three $Cl^-$ ions outside the coordination sphere satisfy the primary valency of Co (+3) and ionise freely in solution.
Step 4: What satisfies secondary valency?
Secondary valency is satisfied by both neutral molecules (like $NH_3$, $H_2O$, $en$) AND negative ions within the coordination sphere. This corresponds to the coordination number.
Step 5: Evaluate the options.
Option 1: "neutral molecules or negative ions" satisfy primary valency - Wrong (neutral molecules satisfy secondary valency). Option 2: primary valency equals coordination number - Wrong (this is secondary valency). Option 4: primary valency is non-ionisable - Wrong (it is ionisable by definition). Option 3: primary valency is satisfied by negative ions - Correct!
Step 6: State the final answer.
According to Werner's theory, primary valencies are satisfied by negative ions (anions) only. \[ \boxed{\text{Primary valencies are satisfied by negative ions only}} \]