The preposition "along" is used to describe movement or direction parallel to the edge of something, such as a river.
The correct sentence should be:
"He walked along the bank of the river."
Thus, the correct answer is Option B ("along").
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?