There is a deposit that can react with hydrochloric acid completely to generate gas, which proves that this deposit is BaCO3, not BaSO4. It shows that there must be carbonate and barium salt in white solid (that is, there must be Na2CO3 and BaCl2), and there must be Na2SO4.

And in the following answer:

B Na2SO4 may exist (then acid-insoluble BaSO4 may be generated, wrong).

C. there is definitely no BaCl2 (there is definitely no precipitation. Error)

D. Only Na2CO3 (then no precipitate is generated. Error)

By elimination, the only obvious answer is a.

And what the landlord thinks is reasonable. Theoretically, when hydrochloric acid is just added, the CO2 produced is directly proportional to the amount of hydrochloric acid added, indicating that there should be no NaOH. If there is NaOH, a certain amount of hydrochloric acid will be consumed, so the relationship between the amount of hydrochloric acid added dropwise and the amount of CO2 will not be like this.

But beyond that, there is no answer.

If you have to choose one, only A is close to the right, you can only assume that the amount of NaOH is very small and ignore the consumption of HCl.

Personally, I think there are some objections to this issue, so we can discuss it.