The most stable ion is [Fe(CN)6]3-

Explanation:
1. Introduction:
In this question, we need to identify the most stable ion among the given options. Stability of an ion depends on various factors such as the nature of the ligands attached to the central metal ion, the charge on the ion, and the overall geometry of the ion.

2. Analysis of options:
Let's analyze each option to determine the stability of the ions:

a) [Fe(OH)5]3-:
This ion has five hydroxide (OH-) ligands attached to the central iron (Fe) ion. Hydroxide is a weak ligand, and it can easily dissociate from the metal ion. Therefore, this ion is less stable compared to other options.

b) [Fe(Cl)6]3-:
This ion has six chloride (Cl-) ligands attached to the central iron (Fe) ion. Chloride is a moderate to strong ligand, and it forms relatively stable complexes with metal ions. However, it is not as strong as cyanide (CN-) ligand.

c) [Fe(CN)6]3-:
This ion has six cyanide (CN-) ligands attached to the central iron (Fe) ion. Cyanide is a strong ligand, and it forms highly stable complexes with metal ions. The strong bond between the cyanide ligands and the iron ion contributes to the stability of this ion. Therefore, [Fe(CN)6]3- is the most stable ion among the given options.

d) [Fe(H2O)6]3-:
This ion has six water (H2O) ligands attached to the central iron (Fe) ion. Water is a weak to moderate ligand, and it forms relatively stable complexes with metal ions. However, it is not as strong as cyanide (CN-) ligand.

3. Conclusion:
Based on the analysis, the most stable ion among the given options is [Fe(CN)6]3-. The strong bond between the cyanide ligands and the iron ion contributes to its stability.