The correct order of metal-carbon distance is

Concept:

→ Metal carbon distance for metallocenes depends upon the presence of unpaired electrons.

→ As the number of unpiared electrons for a system increases, metal carbon bond length also increases.

Explanation:

→ Ni(η5 − Cp)2 have 20 electrons (TVE = 10 × 1+ 2 × 5)

→ Co(η5 − Cp)2 have 19 electrons (TVE =  9 × 1+ 2 × 5)

→ Fe(η5 − Cp)2 have 18 electrons (TVE =  8 × 1+ 2 × 5)

Out of these electrons 12 electrons are involved in ligand group orbitals means that 12 of the valence electrons in the ligands are involved in bonding to the metal ion through their group orbitals because metals presentt in above complexes are in +2 oxidation state.

The remaining electrons would be metal-centered electrons, which are typically involved in d-orbital hybridization and contribute to the metal-ligand bonding.. Thus,

Ni(η5 − Cp)2 have 8 electrons  ( 20 - 12 electrons = 8 electrons)

Co(η5 − Cp)2 have 7 electrons ( 19 - 12 electrons = 7 electrons)

Fe(η5 − Cp)2 have 6 electrons  (18 - 12 electrons = 6 electrons)

Arrangements of these electrons is as:

Ni(η5 − Cp)2 have two unpaired electrons.

Co(η5 − Cp)2 have one unpaired electron.

Fe(η5 − Cp)2 ​have one unpaired electron.

Thus, the order of metal c arbon bond is Ni(η5 − Cp)2 > Co(η5 − Cp)2 > Fe(η5 − Cp)2.

Conclusion:
The correct answer is option 4.