A. Liquid A became acidic first.
Step-by-step explanation:
Here, we have 2 liquids, liquid A and liquid B.
Liquid A is just distilled water.
Liquid B is distilled water with crushed antacid tablet (base) added to it.
If we add vinegar that is an acid to both the liquids.
Liquid A becomes acidic since it has only distilled water.
Liquid B becomes neutral as it has antacid tablets (base) dissolved in distilled water, and so acid reacts with base and gets neutralized.
So A is the answer.