To protect tooth enamel from dissolving, it is better to keep mouth pH above which limit?

Correct answer: About 5.5

The governing concept is acid attack on tooth enamel. Enamel is mainly a calcium-containing mineral, and acids can dissolve it when the mouth becomes sufficiently acidic. In school-level chemistry, the commonly cited critical pH is about 5.5; below this value, demineralisation of enamel becomes more likely and repeated exposure can contribute to tooth decay. Therefore the mouth should preferably remain above about 5.5, making option B correct. The other values are far more acidic and would not provide protection.