1. Identify the complex ion

   The compound is $K_3[Fe(CN)_6]$

   Here, the coordination entity is $[Fe(CN)_6]^{3-}$

   and there are $3$ potassium ions: $3K^+$

2. Find the oxidation state of iron

   Let the oxidation state of Fe be $x$.

   Each cyanide ligand $CN^-$ has charge $-1$. Since there are $6$ cyanide ligands, total ligand charge is: $6(-1)=-6$

   The charge on the complex ion is $-3$, so $x-6=-3$ $x=+3$

   Thus, iron is in the $+3$ oxidation state.

3. Name the ligands

   The ligand $CN^-$ is named cyanido (older form: cyano). In standard option-based JEE naming, hexacyano is commonly used.

   So the complex part is named: $\text{hexacyanoferrate(III)}$

4. Why ferrate and not iron?

   Since the complex ion is anionic, $[Fe(CN)_6]^{3-}$ the metal name ends with -ate.

   For iron, the anionic complex name becomes ferrate.

5. Name the cation first

   The cation is $K^+$, so we write potassium first.

   Therefore, the full IUPAC name is: $\boxed{\text{Potassium hexacyanoferrate(III)}}$

6. Check the options

   • A: Potassium hexacyanoferrate (II) $\rightarrow$ wrong oxidation state
   • B: Potassium hexacyanoferrate (III) $\rightarrow$ correct
   • C: Potassium hexacyanoiron (II) $\rightarrow$ wrong because anionic complex uses ferrate, not iron, and oxidation state is also wrong
   • D: Tripotassium hexcyanoiron (II) $\rightarrow$ incorrect naming and wrong oxidation state

Hence, the correct option is B.