Ans. is Ti4+.Its because cation is formed by loosing electron and anion is formed by gaining electron.And cation will be smaller than its parent atom and its valence electron is more tightly bounded while anion is larger than its parent atom and its electron is bounded with same energy as that of the parent atom.So cation with greater positive charge will be very small compared to other elements.

in isoelectronic species size is inversely proportional to no of protons

cation are smaller than parent atom. more the cationic charge smaller will be the size of ion

it is ti

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answer is 4th option in isoelectronic species group the smallest size is that ion which has more positive size

For isoelectronic species higher the charge on the cation smaller is the size.