Which of the following ion has smallest ionic radius?
Ti4+
Lets calculate the number of electrons :
K+ → 18 electrons
Ca2+ → 18 electrons
Ti3+ → 18 electrons
Ti4+ → 18 electrons
So, therefore, they all are isoelectronic species.
What do we know about isoelectronic species?
Higher the ionic charge on the atom more will be the ionic radius.