The correct answer is A. treated.
The other options are not as accurate because they do not convey the same sense of respect and consideration that is implied in the word “treated.” For example, “regarded” means to look at or consider something in a particular way, but it does not necessarily mean to treat it with kindness or respect. “Behaved” means to act in a particular way, but it does not necessarily mean to act in a way that is beneficial or helpful to others. And “cared” means to feel concern or interest in someone or something, but it does not necessarily mean to take action to help or protect them.
In the context of the question, the firm is being described as treating its workers well. This means that the firm is behaving in a way that is respectful and considerate of its workers, and that it is taking action to help and protect them. The other options do not convey this same sense of respect and consideration, and so they are not as accurate.