Final answer:
Arsenic is considered a Division 1 toxic substance due to its high toxicity levels, which can cause severe health problems and have been historically recognized for its use as a poison.
Step-by-step explanation:
Arsenic is a naturally occurring element that can be found throughout our environment, including in water. While arsenic is potentially an essential trace element for animals, even in small amounts, it is known for its toxicity. In fact, a toxic dose of arsenic corresponds to about 7,000 ppb and higher. Considering the severity of health issues related to arsenic poisoning, such as those suffered by millions in Bangladesh due to contaminated water, and its historical use as a poison, it is safe to categorize arsenic as a Division 1 toxic substance. Therefore, the correct answer is 1) toxic.