Which of the following compounds is a weak acid and commonly acts as a color indicator?

Phenolphthalein is indeed the correct choice because it is a common weak acid that is widely used as a pH indicator in titrations and other chemical analyses. Its structure allows it to change color in response to pH changes: it usually appears colorless in acidic solutions and turns pink as the solution becomes more basic. This color change is utilized to determine the endpoint of acid-base reactions.

The other compounds listed do not serve this specific function. Sodium bicarbonate is a salt that acts as a weak base rather than an acid. Hydrazine is a strong reducing agent and does not demonstrate acidic properties nor is it used as an indicator. Ammonium sulfate, while it has acidic properties as a salt, does not function as a pH indicator either. Thus, phenolphthalein is distinguished by its role in acid-base chemistry and its color-changing ability, making it the correct answer.