The most stable ion among the options given is (c) [Fe(CN)6]3-.

Explanation:

Stability of Ions:

The stability of an ion is determined by various factors such as charge, size, and the nature of the ligands surrounding the central metal ion.

Charge:

In general, ions with higher charges are less stable due to increased electrostatic repulsion between the charged species. Therefore, an ion with a lower charge would be more stable.

Size:

The size of the ion also plays a role in its stability. Generally, smaller ions are more stable because they can form stronger bonds with the surrounding ligands.

Nature of Ligands:

The nature of the ligands surrounding the central metal ion also affects the stability of the ion. Ligands can be classified as weak field or strong field based on their ability to form strong bonds with the metal ion.

Analysis of Options:

(a) [Fe(OH)3]3-:
- This ion has a charge of -3.
- The hydroxide ligands (OH-) are weak field ligands.
- Although the charge is low, the weak field ligands may result in a less stable ion compared to other options.

(b) [Fe(Cl)6]3-:
- This ion has a charge of -3.
- The chloride ligands (Cl-) are weak field ligands.
- Similar to option (a), the weak field ligands may result in a less stable ion.

(c) [Fe(CN)6]3-:
- This ion has a charge of -3.
- The cyanide ligands (CN-) are strong field ligands.
- The strong field ligands can form stronger bonds with the central metal ion, resulting in a more stable ion.

(d) [Fe(H2O)6]3:
- This ion has a charge of +3.
- The water ligands (H2O) are weak field ligands.
- The positive charge on the ion makes it less stable compared to the negatively charged options.

Conclusion:

Among the given options, (c) [Fe(CN)6]3- is the most stable ion. The strong field cyanide ligands form stronger bonds with the central iron ion, leading to increased stability. The other options have weak field ligands or higher charges, which make them less stable.