A or B but i'm leaning more towards A
Double replacement.
Ba(OH)2 + H2SO4 ====> 2H2O + BaSO4
You didn't ask a question just to let you know
A. Ammonium = polyatomic ion
B. Oxygen = compound
C. Silver = element
D. Nitrate = polyatomic ion
E. Helium = element
F. Water = polyatomic ion
G. Nitrogen = element
H. Carbon dioxide = compound