Explanation:

The stability of low oxidation states of transition metals is determined by the strength of the ligand-field stabilization. Ligands that have a strong ability to donate electron density to the metal center can stabilize the low oxidation states. In the case of tungsten (W), the ligand that best stabilizes its low oxidation state is NH3 (ammonia).

Answer: NH3

Reasoning:
The stability of low oxidation states of tungsten can be explained based on the spectrochemical series, which ranks ligands according to their ability to split the d-orbitals of the metal ion. In general, ligands that are strong field ligands, i.e., those that produce a large splitting of the d-orbitals, are better able to stabilize low oxidation states.

Comparison of ligands:

1. H2O (water):
Water is a weak field ligand and has a relatively low ability to donate electron density to the metal center. It cannot produce a large splitting of the d-orbitals, and therefore, it is not very effective in stabilizing low oxidation states of tungsten.

2. NH3 (ammonia):
Ammonia is a stronger field ligand compared to water. It can donate electron density to the metal center through its lone pair of electrons. This leads to a larger splitting of the d-orbitals, making ammonia more effective in stabilizing low oxidation states of tungsten.

3. CO (carbon monoxide):
Carbon monoxide is a stronger field ligand compared to both water and ammonia. It is a π-acceptor ligand, meaning it can donate electron density through its π-bonding orbital and accept electron density into its π*-antibonding orbital. This results in a significant splitting of the d-orbitals, making carbon monoxide even more effective in stabilizing low oxidation states of tungsten.

4. F- (fluoride):
Fluoride is a very strong field ligand. It is a small and highly electronegative atom, which leads to a significant splitting of the d-orbitals. However, fluoride is not the best ligand for stabilizing low oxidation states of tungsten because it is a weak donor of electron density compared to CO and NH3.

Conclusion:
Among the given options, NH3 (ammonia) is the ligand that best stabilizes the low oxidation state of tungsten. It is a stronger field ligand compared to water and a weaker field ligand compared to carbon monoxide and fluoride.