Answer:
a. sulfur and oxygen
b. sodium and calcium
d. oxygen and chlorine
Explanation:
The ionic compounds have to be made with element with positive charge(metal) and element with negative charge(gas). Both sulfur and oxygen is gas with negative charge. Both sodium and calcium is metal with positive charge. Chlorine also gas, so it can't make ionic compound with oxygen.
Sodium and sulfur is the only option that can make ionic compound since Sodium is metal and sulfur is gas.
