The phrase "500 dollars a month" refers to a specific amount of money. When referring to amounts, prices, or quantities, we do not use an article before the number.
"I live on 500 dollars a month."
Thus, the correct answer is (D) No article required.
Arun’s present age in years is 40% of Barun’s. In another few years, Arun’s age will be half of Barun’s. By what percentage will Barun’s age increase during this period?