Answer: option C. keep materials in unlabeled containers.
Justification:
Indeed, keeping materials in unlabeled containers is a risky, dangerous, bad practice.
Labels should be according to international standards. That is, further to the name of the product, labels must signal the danger associated with the product: whether it is poison, toxic, flammable, acid, caustic, explosive, or harmful to the body or the enviroment.
On the other hand, the other options, a. mix chemicals in well ventilated areas, b. follow the instructions on the label, and d. use chemicals in well ventilated areas, are good safety practices, among many others.
