Final answer:
The law that required truthful labels was the Pure Food and Drug Act. It established federal standards of inspection and sanitation on meatpackers and required drug makers to list ingredients.
Step-by-step explanation:
The law that required truthful labels was the Pure Food and Drug Act. This law was enacted in 1906 and it established federal standards of inspection and sanitation on meatpackers. It also required drug makers to list ingredients on their products. The Pure Food and Drug Act was inspired by various exposés on adulterated foods and dangerous patent medicines.