The preposition "with" is used to indicate the instrument used to perform an action.
In the given sentence, a stick is the instrument used to beat someone.
Thus, "with" is the correct preposition.
"He was beaten with a stick."
Thus, the correct answer is "with" (Option B).
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?