Explanation

(b)
The given substances react with water in the following manner
$\ \ \ \ \ \ \ \ \ \ \ \ Na + H_{2}O\ \longrightarrow NaOH + H_{2}(g)$
$2Na_{2}O_{2} + 2H_{2}O\ \longrightarrow 4NaOH + O_{2}(g) + H_{2}(g)$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ Ca + H_{2}O \longrightarrow CaO + H_{2}$
$\ \ \ \ \ \ CaH_{2} + 2H_{2}O\ \longrightarrow Ca\ (OH)_{2} + 2H_{2}$
$\ \ \ \ \ \ \ \ \ \ \ \ CaO + H_{2}O \longrightarrow Ca(OH)_{2}$
$\ \ \ \ \ \ \ CaC_{2} + 2H_{2}O\ \longrightarrow Ca(OH)_{2} + C_{2}H_{2}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ Ba + 2H_{2}O \longrightarrow Ba(OH)_{2} + H_{2}$
$\ \ \ \ \ \ \ \ \ \ \ BaO_{2} + H_{2}O \longrightarrow Ba^{2 +} + O_{2} + H_{2}O_{2}$
Hence, Ca and $CaH_{2}$ is the pair that gives same gaseous product on reaction with water