First of all, let's calculate the polarity of every molecule.
To calculate the polarity we need to know the electronegativity of every element too.
To know the polarity, we proceed like this:
a) Ca2O3
The absolute value of the difference in electronegativity (ΔEN). If it is higher than 1.7 or equal to 1.7 we have an ionic compound.
ΔEN = EN (O) - EN (Ca) = 3.44 - 1 = 2.44 > 1.7 => Ionic
Ypu can find out electronativities from the periodic table
b) CaCl2
ΔEN = 3.16 (Cl) - 1 (Ca) = 2.16 (ionic)
c)RbBr
ΔEN = 2.96 (Br) - 0.82 (Rb) = 2.14 (ionic)
d)MgCl2
ΔEN = 3.16 (Cl) - 1.31 (Mg) = 1.85 (ionic)
e)K2S
ΔEN =2.58(S) - 0.82 (K) = 1.76 (ionic)
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The formula is incorrect for Ca2O3 because the oxidation state for Ca is +2, and the ionic compound is CaO, not Ca2O3
Answer: a) Ca2O3