Which of the following is not isoelectronic with the others?
Two or more molecular entities (atoms, molecules, ions) are described as being isoelectronic with each other if they have the same number of electrons and the same structure (number and connectivity of atoms), regardless of the nature of the elements involved.
Sc^3+ has 18 electrons
V^4+ has 19 electrons
Cl^- has 18 electrons
Si^4- has 18 electrons
Ar has 18 electrons
All the ions and element (Argon) have 18 electrons except for Vanadium ion, which has 19 electrons.
Therefore...V^4+ is not isoelectronic with the others.