Answer:
To check whether a chemical equation is balanced or not we usually look if elements on the product side are numerically equal to the reactant side or not.
A. Na + MgCl₂ → NaCl + Mg
Reactant Product
Na = 1 1
Mg = 1 1
Cl = 2 1
Unbalanced
B. 2LiOH + H₂S → Li₂S + 2H₂O
Reactant Product
Li = 2 2
S = 1 1
O = 2 2
H = 4 4
Balanced
C. AgNO₃ + NaCl → NaNO₃ + AgCl
Reactant Product
Ag = 1 1
N = 1 1
Na = 1 1
Cl = 1 1
O = 3 3
Balanced
D. 3PBr₃ + 2Cl₂ → 3PCl₃ + 2Br₂
Reactant Product
P = 3 3
Br = 9 4
Cl = 4 9
Unbalanced
Therefore, equations B and C are balanced chemical equations.