When heating a substance, a phase change will start to occur when the kinetic energy of the particles is the attractive forces between the particles.

equal to

low enough to overcome

great enough to overcome

great enough to combine

low enough to combine

First let's find out the oxidation number of Fe in K₄[Fe(CN)₆] compound.

The oxidation number of cation, K is +1. Hence, the total charge of the anion, [Fe(CN)₆] is -4. CN has charge has -1. There are 6 CN in anion. Let's assume the oxidation number of Fe is 'a'.

Sum of the oxidation numbers of each element  = Charge of the compound
                                                          a + 6 x (-1) = -4
                                                                     a -6 = -4
                                                                         a = +2

Hence, oxidation number of Fe in [Fe(CN)₆]⁴⁻ is +2. 

Now Fe has the atomic number as 26. Hence, number of electrons in Fe at ground state is 26. 
Electron configuration = 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s² = [Ar] 3d⁶ 4s²

When making Fe²⁺, Fe releases 2 electrons. Hence, the number of electrons in Fe²⁺ is 26 - 2 = 24.
Hence, the electron configuration of Fe²⁺ = 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶
                                                                      = [Ar] 3d⁶

Hence, the number of 3d electrons of Fe in K₄[Fe(CN)₆] compound is 6.