Concept:
Poisons are substances that can cause injury, illness, or death when introduced into the body. In forensic toxicology, certain poisons are historically significant because of their frequent use in crimes.
Step 1: Understanding the historical importance of arsenic.
Arsenic has been widely used as a poison throughout history due to several reasons:
- It is colorless, tasteless, and odorless.
- It can be easily mixed with food or drink.
- Symptoms often resemble common illnesses, making detection difficult in earlier times.
Step 2: Why it is called the "King of Poisons".
Because of its frequent use in historical poisoning cases and its high toxicity, arsenic became known as the:
\[
\boxed{\text{King of Poisons}}
\]
Thus, the correct answer is
Arsenic.