The correct articles here are "the" for both blanks. The phrase "world champion" is specific and refers to a definite person, so "the" is required before it. Additionally, "the third year" refers to a specific, known event or time period, so "the" is also appropriate.
Thus, the correct phrase is: "She is the world champion for the third year in succession."
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?