The word "east" refers to a specific direction that is universally known.
We use "the" before singular, countable nouns when referring to something specific or commonly known.
Thus, the correct sentence is:
"The Sun rises in the east."
Thus, the correct answer is (C) "the".
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?