Answer:
SrF2>SrO>RbF>CsBr
Explanation:
Ionic solids have their melting points dependent on:
i) size of the the ion: the smaller the size of the ion, the more closely packed the ionic sphere will be and the greater the melting point.
ii) Charge on the ion, the higher the charge on the ion the greater the electrostatic attraction between the ions and the greater the melting point.
In the arrangement above, the order of cation sizes are Sr2+<Rb+<Cs+
the order of anion sizes are F-<O2-<Br-
This explains the order of melting points given in the answer.
