The correct answer is A. Early in the twentieth century, the Siberian tiger was added to the endangered species list.
Explanation:
In the English language, the word "endangered" is an adjective used to describe a species that is in danger because it might disappear or face extinction. For example, the Sumatran elephant is an endangered species. This implies this word is used mainly to refer to species and also it can only be placed before a noun to describe it as it is an adjective.
According to this, the sentence that uses this word correctly is "Early in the twentieth century, the Siberian tiger was added to the endangered species list" because in this sentence the word "endangered" refers to a species of tiger that might face extinction and it is correctly used as an adjective.
