Answer
a) Li & Br will form ionic compound.
b) Cl & O will not form ionic compound.
c) Ca & Mg will not form ionic compound.
d) Ba & Cl will form ionic compound.
Work Step by Step
Ionic compound is formed when one atom loses one or more electrons to form cation and another atom gain that electron(s) to form anion.
Here, Li has one electron in the outermost shell. So, it loses electron to form $Li^{+}$ and Br gains that electron to form $Br^{-}$. So, Li & Br will form ionic compound.
Cl & O both are non-metal. So, they both can gain electron to make their electronic configuration stable. Losing electron is highly unfavorable for both the atoms. So, cation is not formed. That's why Cl & O will not form ionic compound.
Ca & Mg both are metal. So, they both can lose electron to make their electronic configuration stable (nearest noble gas electronic configuration). Gaining electron is highly unfavorable for both the atoms. So, anion is not formed. That's why Ca & Mg will not form ionic compound.
Ba is a metal. So, it can lose electron to form $Ba^{2+}$ ion. Cl is a non-metal. So, it can gain electron to form $Cl^{-}$ ion. So, Ba & Cl will form ionic compound.
