Answer is option 4 -CO
because CO is a strong field ligand. it has strong sigma donor and pi acceptors. these strong field ligands can stabilise the low negative oxidation state.....

Stabilization of Negative Oxidation States of d-Block Elements by Ligands

Introduction
The d-block elements exhibit a variety of oxidation states, including negative oxidation states. However, the stability of these negative oxidation states is often poor due to the high energy required to remove electrons from the d orbitals. Ligands can help to stabilize negative oxidation states by interacting with the metal cation and donating electrons to the d orbitals.

Types of Ligands
There are several types of ligands that can stabilize negative oxidation states of d-block elements. These include:

1. Fluoride ion (F-)
The fluoride ion is a strong Lewis base that can donate electrons to the d orbitals of a metal cation, stabilizing negative oxidation states. For example, in the compound K2MnF6, the fluoride ion stabilizes the Mn(IV) oxidation state.

2. Water (H2O)
Water is a weak Lewis base that can also donate electrons to the d orbitals of a metal cation. However, its ability to stabilize negative oxidation states is weaker than that of stronger ligands such as F-. For example, in the compound [Fe(H2O)6]2+, the water ligands stabilize the Fe(II) oxidation state.

3. Ammonia (NH3)
Ammonia is a stronger Lewis base than water and can donate electrons more effectively to the d orbitals of a metal cation. For example, in the compound [Co(NH3)6]3+, the ammonia ligands stabilize the Co(III) oxidation state.

4. Carbon Monoxide (CO)
Carbon monoxide is a very strong Lewis base that can donate electrons to the d orbitals of a metal cation even more effectively than ammonia. For example, in the compound [Fe(CO)5]^-2, the carbon monoxide ligands stabilize the Fe(-I) oxidation state.

Conclusion
In summary, ligands can stabilize negative oxidation states of d-block elements by donating electrons to the d orbitals of a metal cation. Stronger ligands such as fluoride ion and carbon monoxide are more effective at stabilizing negative oxidation states than weaker ligands such as water and ammonia.