Answer:
See explanation
Step-by-step explanation:
The ionic radius of metal M decreases as the magnitude of charge on the metal M increases. The ionic radius is the distance between the nucleus and the electron in the outermost shell of an ion. So, ionic radius decreases with increasing positive charge.
We can see that from the various formulae in the question that the metal forms cations M^2+, M^3+ and M^4+ respectively. The order of decreasing ionic radius of the compounds is;
MX2 > MX3 > MX4