All of the following substances react with water. The pair that gives the same gaseous product is 

$$\begin{gathered} \mathrm{Ca}\text{ and }{\mathrm{CaH}}_2\text{ both react with }{\mathrm{H}}_2\mathrm{O}\text{ to form }{\mathrm{H}}_2\text{ gas, } \\ \mathrm{Ca}+2{\mathrm{H}}_2\mathrm{O}⟶\mathrm{Ca}(\mathrm{OH}{)}_2+{\mathrm{H}}_2 \\ {\mathrm{CaH}}_2+2{\mathrm{H}}_2\mathrm{O}⟶\mathrm{Ca}(\mathrm{OH}{)}_2+2{\mathrm{H}}_2 \\ \text{ whereas }\mathrm{K}\text{ gives }{\mathrm{H}}_2\text{ while }{\mathrm{KO}}_2\text{ gives }{\mathrm{O}}_2\text{ and }{\mathrm{H}}_2{\mathrm{O}}_2 \\ 2\mathrm{K}+2{\mathrm{H}}_2\mathrm{O}⟶2\mathrm{KOH}+{\mathrm{H}}_2 \\ 2{\mathrm{KO}}_2+2{\mathrm{H}}_2\mathrm{O}⟶2\mathrm{KOH}+{\mathrm{O}}_2+{\mathrm{H}}_2{\mathrm{O}}_2 \\ \text{ Similarly, Na gives }{\mathrm{H}}_2\text{ while }{\mathrm{Na}}_2{\mathrm{O}}_2\text{ gives }{\mathrm{H}}_2{\mathrm{O}}_2 \\ 2\mathrm{Na}+2{\mathrm{H}}_2\mathrm{O}⟶2\mathrm{NaOH}+{\mathrm{H}}_2 \\ {\mathrm{Na}}_2{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}⟶2\mathrm{NaOH}+{\mathrm{H}}_2{\mathrm{O}}_2 \\ \text{ Likewise }\mathrm{Ba}\text{ gives }{\mathrm{H}}_2\text{ while }{\mathrm{BaO}}_2\text{ gives }{\mathrm{H}}_2{\mathrm{O}}_2 \\ \mathrm{Ba}+2{\mathrm{H}}_2\mathrm{O}⟶\mathrm{Ba}(\mathrm{OH}{)}_2+{\mathrm{H}}_2 \\ {\mathrm{BaO}}_2+2{\mathrm{H}}_2\mathrm{O}⟶\mathrm{Ba}(\mathrm{OH}{)}_2+{\mathrm{H}}_2{\mathrm{O}}_2 \end{gathered}$$