I think that is c i hope that it help you out
First you need to balance the equation. Ba(OH)2+2HNO3=Ba(NO3)2+H2O. Then you can get the ratio of mole number of the reactants. The ratio is Ba(OH)2:HNO3=1:2. So the molarity of Ba(OH)2 is 38.5*0.85/2/20=0.82 molar.
The ball will float as it’s density is less than the salt water.