Why B?

When you're answering a principle question, the conclusion of your answer choice can never assume what is sufficient in the argument. If you diagram the argument you will see that the necessary outcomes (for regular and contrapositive) are that incompetence is a potential threat to society, and that they should not have to get a technical license. Answer B correctly concludes the arguments necessary contrapositive

B concludes that designers shouldn't be subjected to a licensing requirement and states that there aren't circumstances in which poor work would cause a threat to safety or health, so it's perfect. The stimulus can be diagrammed as Not incompetence poses threat-->no licensing requirement (licensing requirement-->incompetence poses threat) B is something we can infer from this conditional statement.