The phrase "day" refers to an unspecified or non-specific day. In such cases, the correct article to use is "a" (indicating any general day).
The phrase "smile" refers to a non-specific smile. Since "smile" begins with a consonant sound, the correct article is "a" instead of "an".
The correct answer is (B) a, a.
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?