If we have in mind the following procedure:
BACo3(s) + H +1 -> Ba +2 + HCO3 -1
CO3 - 2 + H+1 -> HCO3 -1
As nitric acid is added then {CO3 -2 } decreases meaning that more BaCo3 can dissolve. The graph you need to use is the one that the curve goes down and closer to solubility and goes up on conc.
It has a definite volume but not a definite shape
B. Object B is the right choice