Cr 3+

It has 3 unpaired electrons and according to the Crystal Field Theory it has configuration as t2g³ and is extremely stable in aqueous solution!

Mn3+ is strong oxidising agent because  Mn3+  can easily be converted into  Mn2+ having d5 configuration which is stable half-filled configuration.

The most stable ion in aqueous solution among the given options is Cr3+.

Explanation:
Ionic stability is determined by factors such as the charge and size of the ion, as well as the electronic configuration.

1. Charge:
The charge of the ion affects its stability. In general, ions with higher charges are less stable due to increased electrostatic repulsion. Among the given options, Mn2+ has a charge of +2, Cr3+ has a charge of +3, V3+ has a charge of +3, and Ti3+ has a charge of +3. Therefore, all these ions have the same charge of +3, so the charge does not differentiate their stability.

2. Size:
The size of the ion also plays a role in stability. Larger ions tend to be less stable due to weaker attractive forces between the ion and the surrounding water molecules. Among the given options, Mn2+ has an atomic number of 25, Cr3+ has an atomic number of 24, V3+ has an atomic number of 23, and Ti3+ has an atomic number of 22. As we move from Mn2+ to Ti3+, the atomic number decreases, indicating a decrease in size. Therefore, we can infer that Ti3+ is the largest ion among the given options.

3. Electronic Configuration:
The electronic configuration of an ion affects its stability. The electronic configuration of Cr3+ is [Ar]3d3, which is a half-filled d-orbital configuration. Half-filled and fully-filled electron configurations are known to be particularly stable due to the exchange energy associated with electron-electron repulsion. This stability is known as the "extra stability" of half-filled and fully-filled configurations. In contrast, the electronic configurations of Mn2+, V3+, and Ti3+ are [Ar]3d5, [Ar]3d2, and [Ar]3d1, respectively. None of these ions have a half-filled or fully-filled d-orbital configuration.

Therefore, based on the considerations of charge, size, and electronic configuration, we can conclude that Cr3+ is the most stable ion among the given options in aqueous solution.