Answer:
The correct option is: b. pH 6.4-8.0
Step-by-step explanation:
Phenol red is a weak acid that is used as a pH indicator and exists in the form of stable red crystals.
The color of the phenol red solution changes from yellow to red when the change in pH is observed. The color of phenol red transitions from yellow to red when the pH is 6.8 - 8.2 or 6.4 - 8.0
Above the pH of 8.2, the phenol red solution turns a bright pink in color.