Final answer:
Laboratory fermentation tests use a pH indicator to detect B) acid production by bacteria during fermentation, as they typically produce acids that lower the pH of the medium.
Step-by-step explanation:
Laboratory fermentation tests often include a pH indicator because many bacteria produce acids as they ferment carbohydrates. Products such as alcohol and CO2 are also yielded during fermentation processes, but it's the production of acids that results in a decrease of pH in the culture medium, which can be detected by the colour change in a pH indicator like methyl red. For example, bacteria utilizing mixed acid fermentation produce stable organic acid end products, turning a methyl red pH indicator red, whereas bacteria producing alcohols and other less acidic metabolites result in an orange to yellow colour change, which indicates a negative result for acid production.