Answer
(a) $HI(aq) + NaOH(aq) -- \gt NaI(aq) + H_2O(l)$
(b) $HBr(aq) + KOH(aq) -- \gt KBr(aq) + H_2O(l)$
(c) $2HNO_3(aq) + Ba(OH)_2(aq) --\gt Ba(NO_3)_2(aq) + 2H_2O(l)$
(d) $2HClO_4(aq) + Sr(OH)_2(aq) -- \gt Sr(ClO_4)_2(aq) + 2H_2O(l)$
Work Step by Step
1. Write the double replacement equation.
The cation in the acid will always be $H^+$, and, in this case, the anions of the bases are $OH^-$;
2. Balance the equation.
Remember the reaction between an acid and a base with hydroxide ions follows the pattern:
$Acid + Base --\gt Salt + Water$
Where the salt is the ionic compound created by the mixture of the 2 conjugate pairs of the acid and the base.