The correct answer is D. no article.
The word “dollar” is a non-count noun, which means it does not have a plural form and is not preceded by an article. Other examples of non-count nouns include water, air, and love.
In the sentence “I can’t live on 500 dollars a month,” the word “dollars” is used as a measure of quantity, not as a specific number of dollars. Therefore, it is not preceded by an article.
If the sentence were to be changed to “I can’t live on a 500 dollar bill,” then the word “dollar” would be preceded by the article “a” because it is referring to a specific dollar bill.