Final answer:
Ultraviolet light is dangerous for welders because it is high-energy radiation that can penetrate clothing and is invisible to the eye, posing risks of burns, eye damage, and skin cancer.
Step-by-step explanation:
The correct answer to the question is a. Ultraviolet light. While welding, ultraviolet (UV) radiation is a particular concern for safety as it is high-energy radiation that can pass through clothing and cannot be detected by the human eye. This type of electromagnetic radiation can cause harmful chemical reactions in organisms, with the potential to cause skin burns, eye damage such as welder's flash (photo keratitis), and an increased risk of conditions like cataracts and even skin cancer upon prolonged or intense exposure.